[TBP]

The slave states wanted all the salves counted for purposes of apportionment; the free states wanted none. Three fifths was a compromise. It was distasteful but necessary to get the necessary 2/3 (nine) states for ratification.

[Meatspace]

For the purposes of apportionment but not for voting.

[stremba]

Actually it’s really even more cynical than that. All taxes paid to the Federal government had to come from each state in proportion to that state’s population. Northern states wanted slaves to be counted fully for the purpose of taxation but not at all for apportionment of members of the House. Southern states obviously wanted the opposite. Hence the compromise- 3/5 of slave population counted for both purposes.