The Torah in Parashat Re’eh (Debarim 14:28) commands that every three years, one must separate one-tenth of his agricultural produce and give this tithe to the poor. This means that on the third and sixth years of the seven-year Shemita cycle, the Ma’aser Sheni tithe is replaced by Ma’aser Ani – a tithe for the poor. During the other years (first, second, fourth and fifth), after one gives one-tenth of his produce to the Leviyim, he must take a second tithe, bring it to Jerusalem, and eat it there. On the third and sixth year, instead of taking a tithe to Jerusalem, one gives a tithe to the poor.
When a poor person comes to a person’s field to receive Ma’aser Ani, the owner is to give him “Kedeh Sob’o” – an amount that satiates the recipient. The Sefer H’hinuch, in discussing this Misva, specifies the precise amount that must be given for each species of produce. If a large number of poor people come requesting Ma’aser Ani, and the individual does not have enough for them all, then he places the tithe in front of them, and they take for themselves. If a needy man and a needy woman come requesting Ma’aser Ani, then the woman takes precedence, as it is preferable for a woman not to have to go about collecting charity. A person is not permitted to choose a poor person whom he likes and give him his Ma’aser Ani. He is not granted the “Tobat Hana’a” – the right to select somebody for whom he wishes to do a favor and give him the Ma’aser Ani, and must instead make it available for all those in need.
On the level of Torah law, this Misva applies only when the majority of the Jewish Nation lives in the Land of Israel.