On May 1787 in Philadelphia, Pennsylvania, the Four Founding Fathers and 51 other men met for the Constitutional Convention. One of the topics they would be discussing was slavery, if they should count the slaves in the south or not and weather it was fair or not.
The south was arguing that they should count the slaves because they are part of the population, therefor they should get more representatives.
The North was arguing back that the slaves can't vote and don't have a say in what happens in the country so they shouldn't be counted. They thought it was unfair that the south would get more of a voice just because they had slaves.
Both sides finally came to a compromise. The south would count 3/5 slaves instead of all of them because the slaves still live in the south so the states should get a bit more of representation, but they can't vote so they will count 3/5 to cut down the drastic difference and make it more fair.