…shall not eat from the holy things until he is purified… (Leviticus 22:4)
Terumah is the special portion given to the kohanim, from which they may feed their households. Our verse says “any person of the descendants of Aaron,” which includes women. From the fact that kohanim could not feed their wives and daughters from the sacrifices they offered in the Temple, we see that this mitzvah must be speaking of terumah, which could be eaten by entire priestly families, males and females alike.
The basis of this mitzvah is to treat the terumah with the proper esteem. It’s not “just grain” or “just fruit.” It’s something special, as we see from the fact that ritually-impure people could not eat from it.
This mitzvah is referenced in the very first mishna on the very first page of Talmud in Brachos. When determining nightfall so that the Shema might be recited, the mishna gives us the guideline “the time when the kohanim go in to eat their terumah” after being purified.
This mitzvah applies to male and female kohanim in Israel at a time when ritual purity may be observed. It is discussed in the Talmud in tractate Sanhedrin on pages 83a-b, and in Makkos on 14b. It is codified in the Mishneh Torah in the seventh chapter of Hilchos Terumos and it is #136 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.