What was the 3 to 5 ratio for counting slaves?

It was a result of a compromise between the North and the South. The North wanted slaves to be counted for taxation purposes in the South to bring in more revenue for the federal government which was something the South did not want. But the South wanted to count slaves towards in population in terms of representation so they could have more members in the House of Representative which the North did not want. In the end they settled upon the idea that each slave counted as 3/5 of a person, both in terms of taxation and representation.