As he began what would be a 20-year sojourn in exile, Jacob prays to God and promises, "Of all that You give me, I will set aside a tenth for You" (Genesis 28:22). This is the first biblical mention of the mitzvah to donate one tenth or a ma'aser of your possessions. In Temple times, farmers gave a tenth of their produce to the priests and Levites. In fact, all fruits and vegetables grown in Israel today have a symbolic amount removed before they reach the markets to fulfil the laws of ma'aser.
These days, however, the relevant aspect of ma'aser is the obligation to give one tenth of your earnings to the poor. That the rabbis require a minimum level of donation is in keeping with the Jewish concept of charity, tzedakah, which comes from the word tzedek, justice. Giving tzedakah or a ma'aser of your income is what keeps society just and protects against massive economic divisions. Money spent on one's children can be included in one's ma'aser, if otherwise it would be too difficult to reach the full one-tenth.