An infinite combo (also just “infinite”) is a combo that in theory can be continued forever.
Primarily, these spawn from loops that can be repeated any number of times.
Even if they do no damage whatsoever, they can stall until time runs out, which means that even nearly-harmless infinites can destroy the game balance.
Because these combos will destroy the balance that the developers originally wanted to prepare for the game, these can be thought of as a form of bug.
In the same way as death combos, the degree of difficulty tends to be quite high (it does have to escape the developer’s notice, after all), but regardless, it is ill-advised to use these against another player (this is “asking for it”); you are free to enjoy using them against the AI.