The Constitution Convention met between May and September of 1787 to address the problems of the weak central government that existed under the Articles of Confederation.
One thing that was argued at the Constitution Convection was that the small states wanted each state to have the same number of representatives in Congress. The big states wanted representation based on population.
The population of slaves would be counted as 3/5 in total when apportioning Representatives, as well as Presidential electors and taxes. This was called The 3/5 compromise