Rashi on Deuteronomy 24:20/רש"י על דברים כ״ד:כ׳

1לא 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).

2אחריך 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).