We’re going a little out of chronological order here, as Mitzvah #216 comes from Leviticus 19:10 and Mitzvah #217 comes from the previous verse, Leviticus 19:9. (More about this in the next mitzvah; we’ll try not to give too much away before then.)
It is a mitzvah to leave a portion of the field for the needy. This unharvested area is called “peah,” meaning a corner. The Torah instructs landowners to allow the poor to take this produce for themselves. Though the Torah only required us to do this for born Jews and converts (“ger,” translated “stranger,” means a convert), the Sages of the Talmud required that the rule be extended to allow non-Jewish poor to take as well, so there should not be animosity between the Jews and their neighbors. Similarly, the Torah did not specify a particular measure to be left over, but the rabbis instituted a minimum of one-sixtieth of the field. (Every morning, in the blessings on Torah study, we say a passage “These things have no prescribed size…” That is the first mishna in tractate Peah and peah is the first item on the list.)
This mitzvah only applies to crops that are grown for food. The owner of the field does not distribute the peah; rather, the poor come and take it themselves. If the owner forgot to leave the corners for the needy, he would have to make the accidentally-harvested produce available to the poor.
The basis for this mitzvah is that God does good things for us and He desires that we “pay it forward.” If He’s given us a field and a harvest, He doesn’t want us to hoard it. Rather, we should share it with those less fortunate. In this manner, we emulate Him and develop our good traits.
This mitzvah applies to everyone but only in Israel at a time when the Jews live there. It is the subject of tractate Peah in the Mishnah. It is codified in the Shulchan Aruch in Yoreh De’ah 332. It is #120 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #1 of the 26 mitzvos that can only be performed in Israel according to the Steipler Gaon.