The Three-Fifths Compromise dealt with how slaves would be accounted for when determining states' representation in Congress. It was a compromise adopted during the United States Constitutional Convention in 1787.
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The Three-Fifths Compromise was a way of accounting (somewhat) for the population of slaves in states that permitted slavery. For taxation and representation purposes, the question was whether slaves should count in the population figures. (They were not considered voting citizens at that time.) 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.
The 3/5ths compromise was a law passed in 1787 that said that slaves would count as 3/5ths of a person. So if there were 5 slaves, they would be counted as 3 people of the population. If there are 500 slaves in a state and 1000 citizens in that state, the population would be counted as 1300 instead of 1500. The reason this was controversial is because states were debating how they should count the population to base on how many seats the states would get in the House of Representatives. Larger states disliked this and smaller states liked it.
