1
לא תפאר. לֹא תִטֹּל תִּפְאַרְתּוֹ מִמֶּנּוּ (חולין קל"א), מִכָּאן שֶׁמַּנִּיחִין פֵּאָה לָאִילָן: 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).
2
אחריך. זוֹ שִׁכְחָה: 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).