…do not finish reaping the corner of your field… (Leviticus 19:9)
As mentioned in the previous mitzvah, this prohibition is actually stated in the verse prior to the obligation to leave the corner for the poor. So in the Torah it says:
(a) Do not finish harvesting the corner of your field (verse 9), (b) rather, you shall leave it for the poor (verse 10).
I guess that, from a codification standpoint, it flows more logically to say:
(a) One is obligated to leave a corner for the poor, (b) therefore do not finish harvesting that area.
Either way, the bottom line is the same.
The Talmud in Makkos (16b) says that peah is what’s a “lav she’nitak l’asei,” a prohibition the violation of which is remedied by an obligation. So, one may not harvest the entire field (so that the poor can come and take) but if he does, he must then give from the harvested produce to the poor, as we discussed in Mitzvah #216.
As we have stated, the reason for these mitzvos is that God wants us to emulate His traits and to share what He has given us with the more needy.
This prohibition applies to both men and women but only in Israel. It is the subject of the tractate Peah in the Mishna. It is codified in the Shulchan Aruch in Yoreh De’ah 332 and is #210 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos. It is #6 of the 26 mitzvos that can only be performed in Israel according to the Steipler Gaon.