The word "imba" comes from gaming and means that something is very unfair or superior. The meaning of imba is based on the word "imbalanced" which means "not in balance" or "unbalanced".
The word was particularly popular in the World Of Warcraft community, but it has now become strongly established throughout the gaming scene and has even made it into everyday language as an adjective. Here, it means "super" or "very good" in most cases.
In e-sports, imba usually means that something is too strong and needs to be downgraded. This can be classes, weapons, items or abilities that are simply so strong that the whole game dynamic suffers.
However, you can also use it to call a player very good. "Gankmaster4000 is just imba with the kar" could mean a player who is very strong with the sniper rifle, for example. In such a case, of course, it is meant more as a compliment.