Watching the Red Sox / Atlanta game recap and noticed that the catcher has an earpiece.

[Wounded Bear]

4. It's part of the pitch com system. Been in use for 2 or 3 seasons now...

Generally, the catcher has a module with which he can signal the pitcher what pitch to throw. The pitcher has a receiver that fits inside his cap that vocalizes the pitch selection to him. The catcher also has an earpiece so he can hear what he sent to verify it. They can also have one defender other than the pitcher with an earpiece, usually the shortstop.

It replaces the old system of having the catcher signal the pitcher with finger movements between his legs, which can be picked up by a runner on second base, or by someone in the outfield seats, which would be illegal.

Sometimes the pitcher has a transmitter as well, but not many pitchers call their own pitches these days.