We have previously discussed some agricultural gifts that must be left for the poor. These include leket (the gleanings - see Mitzvah #218) and peah (the corners of the field - Mitzvah #216). Here, we are commanded to also leave them shich'cha, a forgotten sheaf.
The reason for this mitzvah is to foster the trait of generosity within us. If God has been kind enough to give us a harvest, what kind of people would we be to hoard it all? God doesn't want us to just be takers, so He gave us mitzvos to accustom us to sharing the goodness He has provided. A fringe benefit of this mitzvah is that God tends to increase His blessings for those who share them.
This mitzvah applies to both men and women wherever and whenever tithes are in effect. It is discussed in the Talmud in tractate Baba Metzia (11a); in the Mishna, in the sixth chapter of tractate Peah. This mitzvah is codified in the Shulchan Aruch in Yoreh Deah 332. It is #122 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #5 of the 26 mitzvos that can only be performed in Israel according to the list of the Steipler Gaon.