The 153rd prohibition is that we are forbidden from eating tevel, i.e. produce from which the terumos1 and ma'asros have not yet been separated.

The source of this prohibition is G‑d's statement2 (exalted be He), "And they shall not desecrate the holy things of the children of Israel that they will separate to G‑d."

One who transgresses this prohibition by eating tevel is punished by a heavenly death penalty. This is alluded to from [the similarity between] this verse, "And they shall not desecrate the holy things," and the verse regarding terumos,3 "And the holy things of the children of Israel they shall not desecrate and [thereby] not die." From the identical expression, "they shall not desecrate," we can derive [the punishment for tevel] from terumah, which is a transgression punishable by death, as we have explained.4

The quote from tractate Sanhedrin5is, "What is the source that teaches us that the punishment for eating tevel is death? From the verse, 'And they shall not desecrate the holy things of the children of Israel that they will separate to G‑d.' This verse speaks about something that 'they will separate' in the future [i.e. tevel].6 And you derive [the punishment for violating] 'they will not desecrate' [regarding tevel] from 'they will not desecrate' written regarding terumah."

Their intention in saying, "in the future," is to say that it is as if the verse reads, "And they shall not desecrate the holy things that they will separate to G‑d in the future." This is the meaning of G‑d's statement (exalted be He), "that they will separate," in future tense, followed by7 the verse, "and they shall bear the guilt of their sin of eating their holy things."

Our Sages said in tractate Makkos,8 "You might think that the only time a person is punished for eating tevel is when nothing at all has been separated. How do we know [that the prohibition applies] when terumah gedolah has been separated, but not terumas ma'aser; when ma'aser rishon has been separated, but not ma'aser sheini, or even9 ma'aser oni? From the verse,10 'You are not allowed to eat in your gates [the ma'aser of your grain, wine or oil].' And later11 it says, '[When you finish taking all the ma'aser from your grain in the third year...] and they will eat in your gates and be satisfied.' Just as later on it refers [even12] to ma'aser oni,13 so too here it refers to ma'aser oni — and the verse says, 'you are not allowed.'"

However, these prohibitions only are punishable by lashes; the heavenly death penalty is only [when the tevel still contains] terumah gedolah or terumas ma'aser, since one who eats ma'aser rishon before the terumas ma'aser has been separated is punishable by death, in G‑d's statement14 (exalted be He) to the Levites, when He commanded them to separate a tithe from their tithe,15 "And the holy things of the children of Israel you shall not desecrate and [thereby] not die.," This is the prohibition not to eat ma'aser rishon when it is still tevel. Therefore, one is punishable by death [for eating it], as explained in tractate Demai.

The summary of all the above: one who eats tevel before the terumah gedolah and terumas ma'aser have been separated is punishable by death, based on the verse, "And they shall not desecrate the holy things of the children of Israel," as we have explained in this mitzvah. One who eats tevel after the terumos have been separated, but before [all] the ma'asros have been separated is punishable by lashes, based on the verse, "You are not allowed to eat in your gates the ma'aser of your grain." You should remember this and not err in it.

The details regarding tevel are explained in many passage of tractate Demai and Terumos, and tractate Ma'asros.