As I tell students, "Bucks have antlers to fight, with other bucks, over girls." This always gets a laugh. Despite all the time I have spent in the woods, I have never seen this in person, but now I have captured it on a trail camera.
It's another similar-sized buck. The two size each other up for a moment...
Once they lock antlers, the goal is to push each other around until one buck decides that the other buck is more powerful and flees.
Most bucks will typically only engage a buck of similar size - a small buck will typically not attempt to fight a much-larger buck, but will submit without conflict. Even if that occurs, the large buck may chase away smaller bucks in order to preserve his own mating opportunities. The small bucks above would probably have fled immediately if confronted by the buck shown below.