Aha, I see where you’re coming from, but if you check the ol’ dictionary, you’ll get a wider scope:
his movements were closely monitored: observe, watch, track, keep an eye on, keep under observation, keep watch on, keep under surveillance, record, note, oversee; informal keep tabs on.
But it’s not strictly limited to vision; so we’re both right! :-)
But they are, in our case, purely for audio purposes.
Another common term is wedges, due to their somewhat wedge-shape design.
Those earphones are called in-ear monitors. They are connected to a wireless belt-pack, which receives yet another special mix of audio. With the proximity of the musicians, one’s monitor mix may be heard by another; in order to get rid of that ‘spill’, in-ears ensure that a certain audio mix is heard only by that musician and none of the others.
To demonstrate what Leonard sees, I’ve taken a sneaky snap before soundcheck.
Hope that helps in the education. :-)