Jamieson, Fausset and Brown on Deuteronomy 26:12-15

12-15. When thou hast made an end of tithing all the tithes of thine increase the third year--Among the Hebrews there were two tithings. The first was appropriated to the Levites (Nu 18:21). The second, being the tenth of what remained, was brought to Jerusalem in kind; or it was converted into money, and the owner, on arriving in the capital, purchased sheep, bread, and oil (De 14:22, 23). This was done for two consecutive years. But this second tithing was eaten at home, and the third year distributed among the poor of the place (De 14:28, 29).