It is a short way of saying they had an "on-again, off-again" relationship, which means that they get together (date) and break up with each other frequently in a cycle.

Hi. An on-off relationship means sometimes two people are having a relationship, then they break up (stop the relationship) then start again. It usually means it's a relationship where the couple can't finish it even though they know it's not working, or can't live without each other. [emoji]