Rashi on Leviticus 19:9/רש"י על ויקרא י״ט:ט׳

1לא תכלה פאת שדך THOU SHALT NOT WHOLLY REAP THE CORNER OF THY FIELD — This means that one must leave פאה (an uncut portion) at the extremity of one's field (cf. Sifra, Kedoshim, Chapter 1 9; Mishna Peah 1:2; Shabbat 23a).

2ולקט קצירך [NEITHER SHALT THOU COLLECT] THE GLEANINGS OF THY HARVEST — Gleanings are ears that drop from the hand of the harvester during the reaping — one or two at a time, but three do not come under the category of לקט (Mishna Peah 6:5; Sanhedrin 99a).