Huh? Isn’t this verse the same one we already used in Mitzvah #216, which told us to leave the corner of the field for the poor?
It sure is. But verse 19:9 has two referents: the corner and the gleanings. Mitzvah #217 told us not to cut the corner and Mitzvah # 219, up next, will tell us not to take the gleanings. When we get to verse 19:10 and it says “leave them for the poor,” the word “them” applies to both the corner and the gleanings, though the obligation to leave them are two separate mitzvos. (As we will see in the next few mitzvos, there are more than just these two mitzvos in those words. We’ll identify the rest as we come to them.)
As far as what “gleanings” are – “leket” in Hebrew – those are dropped crops, which must be left for the needy. If, while harvesting, the farmer drops one or two ears of grain, those are considered leket and may not be picked up. If he drops three or more, it’s not leket and he may pick them up.
The reason behind this mitzvah, as with peah, is to emulate God and share our good fortune with those who may need it.
This mitzvah applies to everyone but only in Israel. It is discussed in the fourth chapter of tractate Peah in the Mishnah. It is codified in the Shulchan Aruch in Yoreh De’ah 332. It is #121 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #2 of the 26 mitzvos that can only be performed in Israel according to the Steipler Gaon.