The northern and southern states disagreed about how population would be counted for taxation and representation purposes -- specifically, whether or not slaves would be counted in southern states' population total.
The Three-Fifths Compromise resolved this disagreement. The Three-Fifths Compromise was a way of accounting (somewhat) for the population of slaves in states that permitted slavery. The Three-Fifths Compromise said that three out of every five slaves could be counted when determining a state's population size for determining how many seats that state would receive in the House of Representatives.
