Deuteronomy 24:20

לא תפאר — This means: thou shalt not entirely remove its (the olive tree’s) glory (תפארת) from it, i.e., its fruit (Chullin 131b). Hence they (the Rabbis) derived the law that one must leave פאה (some quantity of fruit) also on fruit-trees for the poor (cf. Mishnah Peah 4:1). אחריך BEHIND THEE — This implies שכחה (forgotten fruit; that if he forgets to remove some fruit from the orchard, he must leave it for the poor).

כִּ֤י תַחְבֹּט֙ זֵֽיתְךָ֔ לֹ֥א תְפַאֵ֖ר אַחֲרֶ֑יךָ לַגֵּ֛ר לַיָּת֥וֹם וְלָאַלְמָנָ֖ה יִהְיֶֽה׃

When thou beatest thy olive tree, thou shalt not go over the boughs again: it shall be for the stranger, for the fatherless, and for the widow.