what are in

In-Ear Monitors (IEMs), is a type of headphone designed to be partially
inserted into a user’s ear canal by using form-fitting tips often made by a
range materials such as rubber, soft plastic, silicon, and more. It’s also
common to purchase custom molded tips to fit your ear canal. This typically
offers the best results in comfort and noise isolation.

You may notice musicians typically wearing an IEM during a live performance.
This is to listen to a custom set of instruments.

Other names for IEMs are In-Ear-Canalphone, canalphone, or ear-canal headphone.
People believe the acronym IEM became the popular replacement for canalphones
with the iPod generation.

IEMs are sometimes mistakenly called earbuds. The main difference being earbuds
are worn in the opening of the ear. They are not designed to sit in the ear
canal.