MD earphones - question

Often it's the "click track." Like a metronome, it provides the beat to the person listening to it.

Sometimes it's the click track, but more often it's the feed from the Aviom. Each musician has their own Aviom, which allows them to create their own mix to play to. So the Key 3 player or whatever might turn the vocals all the way down so he or she can lock in with the rhythm section, and the French Horn player might turn the other brass instruments up, etc.

So the conductor is going to create their own mix to hear whatever they need to at any given moment, and often change that mix throughout the show.