The Torah commands us to give tithes (10% of agricultural produce) to the poor twice every seven years (in the third and sixth year of every seven-year cycle).
Nowadays, many people give 10% of all their earnings to tzedakah – a common practice that according to many opinions is a binding law.
In addition, every landowner had to give the poor another 1/ 60 of his produce (corn, grapes and olives) in the framework of the commandment of leket, shikheha and pe'ah.
In fact, the leket (single stalks of corn left behind, and single, or small bunches of fruit left on the vine or olive tree), shikheha (single sheaves forgotten in the field) and pe'ah (a part of the field assigned to the poor) did not belong to the landowner; they were the property of the poor.
The landowner only had the right to choose which poor he wanted to give it to.
Moreover, eating a product that has not been tithed is comparable to eating pork.
It turns out, then, that one cannot eat before giving the poor their share.
In other words, I must give because I want to live.
The underlying idea is that giving to the poor, or helping the needy, is not something that the poor and needy need so much as something that must be done by those who have.
Helping others is not seen as a social measure to prevent disasters in the community, but as an obligation imposed on each and every individual.
The poor have the right to demand and receive.
The giver, on the other hand, does not give because he is kindhearted, or because he's under some kind of public pressure, but because it is his duty.
A person must first give, and only then search for needy people to give to.
–Rabbi Adin Steinsaltz
From "Gemilut Hasadim" in A Dear Son to Me by Rabbi Adin Steinsaltz