Our tradition has it that the text of the Chumash was dictated by Hashem to Moshe Rabbeinu word for word and letter for letter. All the other nevi'im were shown a vision , a mareh ha'nevuah, which they interpreted using their own words while under the influence of ruach ha'kodesh. When Moshe Rabbeinu was given a message as a horo'as sho'ah, he was on the same level as all the other nevi'im; but all of the text that pertains to the six hundred and thirteen mitzvos which are binding throughout all the future generations was dictated by Hashem to Moshe word for word.
Regarding this dictation, we are told (Tehillim 62:13) that the pessukim were given to Moshe Rabbeinu with one text, but often had more than one level of interpretation. The most obvious understanding of the passuk is considered peshuto shel mikrah, while the additional level of interpretation is considered part of the Torah sheb'al peh.
Sometimes we have a passuk with more than one interpretation and all of the interpretations are considered peshuto shel mikrah. One such example is, "Lo Sichoneim" (Devarim 7:2). When we enter Eretz Yisroel we will meet up with many different ovdei avodah zorah and we are instructed lo sichoneim. The Gemarah has three interpretations of the verb, "sichoneim", based on three different possible roots of the word, and all three are considered peshuto shel mikrah: 1) the root is chinom (free), and the passuk is saying that we may not give a matnas chinom to an idol worshipper. 2) the root is chein (favor), and we're being told that they should not find chein in our eyes; we may not praise a baseball player or a professor or an artist who worships avodah zorah. 3) the root is chanoya (encampment or settling), and we are commanded that we should not enable the idol worshippers to live in Eretz Yisroel by selling or renting any real estate to them.
There is a difficult passuk (Vayikra 19:26) which prohibits us from eating "al ha'dam". The simple reading of the passuk has one meaning, but the Torah sheb'al peh has additional levels of interpretation which are not l'fi peshuto shel mikrah. The Gemarah (Sanhedrin 63) therefore says that if one violates one of the prohibitions that was taught by one of these additional interpretatoins, there would be no punishment of malkus. Malkus is only for one who violated an aveirah which is similar to, "lo sachsom shor b'disho" (Devarim 25:4), i.e. the prohibition must appear in the Chumash and has to be the understanding of the passuk according to peshuto shel mikrah.
Every so often there will be a passuk where the additional level of interpretation will be just the opposite from the peshuto shel mikrah. In Parshas Naso, (Bamidbar 6:23) the Torah instructs the Kohanim to recite Birchas Kohanim at the conclusion of offering the korbanos tzibbur. The Chumash presents this mitzvah with the two words, "omor lo'hem". The simple translation of the passuk is that they (the Kohanim) should recite the blessing to them, (the tzibbur.) The Torah sheb'al peh has an additional level of interpretation which is just the opposite from the peshuto shel mikrah: the Kohanim are required to offer this blessing only when they (the tzibbur) requested it of them (of the Kohanim). Although this additional level of interpretation is just the opposite of the peshuto shel mikrah, there is no contradiction between the two, and halacha l'maaseh we follow both instructions.
Similarly, the Torah tells us (Devarim 22:3) that one who finds someone else's lost item should publicize the fact that he found it, and he should hold on to it until the owner will come and ask him to return it. The words in the Chumash are, "ad drosh achicha oso". The Torah sheb'al peh has an additional level of interpretation on this passuk which is just the reverse of the peshuto shel mikrah: the one who found the lost article is required to hold on to it until he interrogates the one who claims to be the rightful owner. You must not just take the person's word for it that he is the owner, rather you must ascertain to the best of your ability that he is telling the truth. The additional level of interpretation is just the opposite from the simple reading of the passuk, but halacha l'maaseh there is no contradiction between the two.
The Torah also tells us (ibid 22:4) that if one notices someone else's lost article he may not walk away and act as if he simply did not see it; "You may not make believe that you did not see it" is the simple translation of the passuk. Here, too, the Torah sh'baal peh gives an additional level of interpretation which is just the reverse of the simple meaning; it explains the word "v'hisalamta" to mean "you may ignore the item and make believe that you did not see it". The Gemarah explains that the passuk means to tell us that sometimes you may ignore it, and sometimes not, and then goes into detail to explain under what circumstances is it permissible to make believe that you did not notice the other person's lost item.
We usually assume that the Aramaic Targum Onkelus is only giving peshuto shel mikrah. Rav Soloveitchik explained that Onkelus was an adult when he decided to convert, so he had to sit in class with young children in cheder where the rebbe was teaching Chumash according to peshuto shel mikrah. On the passuk in Parshas Naso, where the Torah sh'baal peh gives the additional level of interpretation (that the Kohanim are only obligated to duchen when the tzibbur requested the beracha from them,) many editions of the Chumash have this additional level of interpretation in the Targum Onkelus, who adds on a word "Kad - when", i.e. when the tzibbur will request a blessing of the Kohanim. In the Sefer Toldos Odom, which is a biography of R' Zelmeleh Volozhiner, the younger brother of R' Chaim Volozhiner, there are several divrei Torah from this prominent student of the Vilna Gaon. He suggests that the word "Kad" that appears in the Targum Onkelus was simply a typo. This passuk happens to be passuk twenty-three in chapter six of Bamidbar. Before the next passuk the typesetter put in the two letters kof and daled to imply that the next passuk is twenty-four. In typesetting every so often letters float from one line to the other; so the letters kof and daled floated into the middle of passuk twenty-three in the Targum Onkelus, and it sounds as if Onkelus is saying that that was the peshuto shel mikrah. The two letters (kof and daled) do not belong in the middle of the passuk because Onkelus is always supposed to be giving the peshuto shel mikrah for young children learning has'cholas Chumash in cheder.
This point that every so often we will have a passuk with more than one level of interpretation is one of the many aspects of the Torah sh'baal peh.