Do not pick your vineyard clean… (Leviticus 19:10)
Yes, it’s yet another referent of the “them” in “you shall leave them.” (If you haven’t already gone and looked up verses 19:9-10 to see how many things are in that phrase, you really should!)
Just as “leket” are dropped crops in the field (see Mitzvos #218-219), “peret” refers to dropped grapes in a vineyard. And, as with leket, we have two mitzvos regarding peret. This mitzvah is the positive one: we are commanded to leave the fallen grapes in our vineyards so that the poor can take them.
This mitzvah applies to everyone but only in Israel. It is discussed in the seventh chapter of tractate Peah in the Mishnah and is codified in the Shulchan Aruch in Yoreh De’ah 332. It is #124 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #4 of the 26 mitzvos that can only be performed in Israel according to the Steipler Gaon.