IEM, or an in ear monitor, is a device that many musicians use on stage. "The most common In Ear Monitor system employs the use of a wireless system to send the mix to the monitors. This system contains a transmitter and a receiver pack that is worn by the performer. (Wikipedia, in ear monitors)
A band member can listen to a mix of what other band members are playing/singing at the moment. Many musicians can be seen adjusting these ear pieces or monitors that are clipped onto their pants during a live performance.
"I had to turn up the volume on my in ear monitor because I couldn't hear Jason's bass very well."