In-ear monitors allow you to do away with stage monitors by using ear buds to monitor what’s happening on stage. The main advantage of using them is that stage volume is reduced (since stage monitors aren’t require anymore) and the performer can have a clean representation of what’s happening on stage regardless of his or her position. However, because in-ear monitors often have sound-isolating technology, the performer will have to rely almost 100% on what he or she is hearing through the earphones. This can be strange, because they will no longer get a good idea of what the room sounds like or how the crowd is reacting. You can easily solve this by having room mics set up for room tone or by removing one of the earbuds.
In-ear monitors can be wireless or wired. It is most common to find them in a wireless format due to the ability to be mobile. In this format, they work in an opposite fashion as a wireless microphone system. From your mixer, you would take the same feed you would send to your stage monitors and plug it in to the wireless monitor transmitter. The transmitter will then send the feed to your wireless receiver belt pack that your headphones are plugged into.