In-ear monitors can be wireless, commonly you will find them in a wireless format due to the ability to be mobile and portable. In this format, they work oppositely as a wireless microphone system. The earpiece is called an in-ear monitor. The in-ear monitor allows the singers or the person onstage to hear what they want.
For example, if you are a singer singing with a band, there is a lot of noise on stage with you. It can be very hard to hear yourself which can make you sing louder and even shout. If you use in-ears, you can have the sound people put only your voice or whatever you may need in your ears. You can choose what you want to hear.