The First Tithe is 10% of a produce which was supposed to be given to a Levite. The Second Tithe was separated from the remaining produce and was brought to Jerusalem Temple during and eaten there. The Third Tithe was given to the poor people in the years when the Second Tithe wasn't required.
The law required certain tithes (or "gifts") from every Jew who owned land. The first tithe was for Levites, the second was for using the temple and the great feasts, and the final tithe was for the poor of the land.