It's another way of saying "fight your own fight" or "do what you do best".
If I'm a slow and plodding fighter that likes to put ppl on the ropes and you're a fighter that likes to move and take the center of the ring, or vice versa, don't get caught up doing what I do.
Many times, when in a fight, one fighter will find themselves fighting at a pace and style that is dictated by their opponent. For some, it's almost a natural thing to do. Instead you want to be the one that is dicatating the where and how of the fight, or minimally not allowing your opponent to do so.
The concept here is that if you do "fight his fight", you are now fighting how he fights best and not how you fight best. This will likely lead to you losing.