If you catch your partner cheating in a dream, it can mean one of several things. Maybe your mind or physical being has seen your partner and caught them in an act, but is suppressing it, for the sake of not upsetting yourself. This is very common because people usually try to repress memories in hope that if your mind doesn’t allow it to exist, you really will believe that it doesn’t exist.
It can also represent the lack of trust you have in your partner. Maybe you’re afraid that they will, or that they are cheating so your mind creates a scenario where you can satisfy your accusations and satisfy your lack of trust in your partner.
Another possibility may be that you’re in a bad relationship, and you’re looking for a way to get out of it, your looking for a reason or a tangible accusation for you to get out of a relationship.
Either way, it’s only a dream, and if your partner didn’t cheat on you in the “real world’ then it shouldn’t be of much concern, but the idea of a cheating partner should raise the question of how you view your relationship with them.