Rashi on Leviticus
לא תעולל AND THOU SHALT NOT GLEAN [THY VINE YARD] — (עלל is a verb connected with the noun עולל "young, tender, not developed"; cf. Samuel 15:3: עולל ויונק) thus לא תעולל means, thou shalt not take the tender grapes of it. They can be recognized as such from the description given in Mishna Peah 7:4 (cf. also Sifra, Kedoshim, Chapter 3 3) where it is stated: What are עוללות? Clusters which have neither כתף, “arms" nor נטף, "drippings".
Rashbam on Leviticus
לא תעולל, meaning something akin to the concept of peyah in cornfields, i.e. the farmer must not harvest the entire vineyard for his own use. The parallel verse appears in Jeremiah 6,9 עולל יעוללו כגפן, “let them glean over and over, as a vine.”