system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When a singer is performing during a concert and they have an earpiece, what are they listening to?

Not sure if this is the best subreddit for this question, but I thought it was appropriate than askreddit.
gpt
It's an in-ear monitor. It helps the singer keep in tune by blocking out external noise and playing back the band audio through the earpiece so the singer can hear themselves in relation to the rest of the band.

Without such monitors, it's actually very difficult for singers to sing in tune because the sound of the band gets drowned out by everyone in the audience and most people can't sing with absolute pitch (i.e. sing the correct frequency/pitch without a reference point).

As an alternative to (or in combination with) in-ear monitors, there often speakers set up on stage that are pointed directly toward the singer/band members that perform a similar function.

In-ear monitors are also sometimes used to secretly provide instructions or other messages to performers.

Edit: It's also worth noting that, in some cases, the audio played back through the in-ear monitor is a pre-recorded version of the song (e.g. from an album). Each band member and/or dancer can then match their live performance to the track played in their ear. This has the added benefit of maintaining consistency (e.g. in tempo and pitch) across performances. This approach also keeps the singer in tune, so long as the band instruments are tuned correctly.