Mishnayos Terumos Perek 1 Mishnah 8

הזיתים הנכתשים – it was customary to crush olives in a mortar-shaped cavity to remove their oil.

תרומתו תרומה – it is completed, from the Torah.

ויחזור ויתרום – from the perspective of the Rabbis, after he has completed the pounding of those olives and the treading of the grapes, he will go back and remove the priest’s due a second time according to the measure that is needed for oil and win that he has removed.

הראשונה מדמעת – if it fell to less than one-hundred [parts] non-holy produce, everything is made subject to the law of Terumah through an admixture and it is prohibited to “foreigners” (i.e., non-Kohanim) and everything is sold to a Kohen – for it is complete priest’s due according to the Torah.

וחייבין עליה חומש – a foreigner (i.e., non-Kohen) who eats it inadvertently pays the principle and [an additional] one-fifth according to the law regarding all foreigners who eat priest’s due.

אבל לא שניה – because it is not other than a Rabbinic ordinance; therefore it is not eaten until priest’s due and tithes are separated from it from another placer, since from the Torah, it is completely produce that cannot be eaten until Levitical and Priestly gifts have been separated, and if you ask – what is the difference here when one separates priest’s due from [olive] oil instead of from pounded olives, that his separated priest’s due is priest’s due, and then separates priest’s due again and what is the difference above concerning one who separates priest’s due olives instead of [olive] oil and grapes instead of wine where his separation of priest’s due is not priest’s due? And one can say that regarding a person who separates priest due from olives instead of [olive] oil, where there is a loss to the Kohen, his separating priest’s due is priest’s due/Terumah, but that the Rabbis are they who decreed that he should go back and separate priest’s due once again.