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."
Term coined by Boston radio talk show host Howie Carr for Thomas Menino, mayor of Boston since 1993. His tounge is way to big for his mouth leading to such phrases (and their meanings):
pottah pot- porta potty
wollin walli- rolling rally
ar chieve- achieve
bee yahn- barn
youse- you (plural)
well...you get the picture