Dv8s
It means you're a cheater.
choko_canyon
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.
Katie M
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.