What does it mean when you cheat on someone in a relationship?

It means you're a cheater.

It doesn't necessarily 'mean' something particular, and even if it does, it means something different depending on the person doing the cheating. It MIGHT mean that the cheater is simply bored with their partner, or it could mean that they're terribly insecure and need sex with others in order to confirm that they're still attractive, or it could mean that they were never taught how to act like a mature adult, or it could mean lots of other things.

It could mean a lot of things.

It could mean your partner isn't meeting your needs

It could mean you don't have moral boundaries

It could mean you are insecure and need more from another person

The only way to really find out is to ask the cheater.