(1) Adonai spoke to Moses, saying: (2) Speak to the whole Israelite community and say to them: You shall be holy, for I, Adonai your God, am holy.
What does it mean to be holy?
(ט) וּֽבְקֻצְרְכֶם֙ אֶת־קְצִ֣יר אַרְצְכֶ֔ם לֹ֧א תְכַלֶּ֛ה פְּאַ֥ת שָׂדְךָ֖ לִקְצֹ֑ר וְלֶ֥קֶט קְצִֽירְךָ֖ לֹ֥א תְלַקֵּֽט׃ (י) וְכַרְמְךָ֙ לֹ֣א תְעוֹלֵ֔ל וּפֶ֥רֶט כַּרְמְךָ֖ לֹ֣א תְלַקֵּ֑ט לֶֽעָנִ֤י וְלַגֵּר֙ תַּעֲזֹ֣ב אֹתָ֔ם אֲנִ֖י יי אֱלֹהֵיכֶֽם׃
(9) When you reap the harvest of your land, you shall not reap all the way to the edges of your field, or gather the gleanings of your harvest. (10) You shall not pick your vineyard bare, or gather the fallen fruit of your vineyard; you shall leave them for the poor and the stranger: I, Adonai, am your God.
לֹ֧א תְכַלֶּ֛ה פְּאַ֥ת שָׂדְךָ֖ - You shall not reap all the way to the edges of your field: This means that one must leave פאה (an uncut portion) at the edges of one's field (cf. Sifra, Kedoshim, Chapter 1 9; Mishna Peah 1:2; Shabbat 23a).
וְלֶ֥קֶט קְצִֽירְךָ֖ Or gather the gleanings of your harvest: Gleanings are ears that drop from the hand of the harvester during the reaping — one or two at a time. (Mishna Peah 6:5; Sanhedrin 99a).
אֲנִ֖י יי אֱלֹהֵיכֶֽם׃ I, Adonai, am your God: the Judge Who is certain to punish if necessary and Who for the neglect of these duties will exact from you nothing less than your souls, as it is said (Proverbs 22:22, 23) "Rob not the poor… for Adonai will plead their cause, and spoil the soul of those that spoiled them." (Sifra, Kedoshim, Chapter 3 7).
The Torah now turns to how we can emulate God’s attributes in practice after having accepted that it is our purpose on earth to emulate God’s characteristics to the extent that God has revealed them to us. We are to perform acts of charity and righteousness. Part of such acts of charity are the providing for the underprivileged out of the bounty God has seen fit to grant us. The specific items known as leket (gleanings)... and pe'ah (edges) are examples of such demonstrations of our generosity towards the poor.
"I, Adonai, am your God" - This recognition of God as our God is demonstrated by our meticulously fulfilling these commandments relating to the time when we gather in our harvest. Before we even give tithes of the completed harvest which has been brought into the barn, we already allow for the poor to help themselves to what other, gentile farmers, might consider the hard-earned product of their own labor.
Maimonides on Pe'ah and Leket (Mishneh Torah: Gifts To the Poor)
1:8) With regard to all of these gifts to the poor, owners may not derive any benefit from them, but rather the poor come and take them regardless of the owner's wishes. Even if he [the farmer] is one of the poor of Israel, they take them from his possession.
2:12) One should only leave pe'ah from the end of one's field so that the poor know the place they are to go and so that it is known to the passers-by, so they will not suspect anything. This is so because there are liars who intend to harvest the whole field who could say to those who see him harvesting the end of the field, "I left my due from the beginning of the field." And moreover [this applies so] he will not wait until a time when no one is around and leaves [his due] to his poor relative. But, if he transgressed and left pe'ah from the beginning of the field or from the middle, this still counts toward [the total amount of] the pe'ah portion. However, he still needs to leave the rest [of his due] from the end of the field so as to fulfill the fitting measure of pe'ah after he has separated the first part.
4:12) One who does not permit the poor to glean or who permits one but prevents another or who assists one of them rather than another is considered a robber of the poor.
4:13) It is forbidden for a man to let loose a lion or something similar within his field so that the poor see it and flee.