With normal use, milk will to cook onto the surface of the steam wand of your espresso machine, so the steam wand should be be wiped down with a towel after each use. This works fine for keeping the outside of the steam wand clean. But over the course of a shift, there may also be some buildup on the inside of the steam wand too. So the inside of the steam wand should be cleaned as part of your normal daily maintenance of your espresso machine. This long, thin brush is made to do that job.
This steam wand brush has a standard, 6 millimeter bore, and it will fit any steam wand. But this particular steam wand is ONE FOOT long with 4" bristles (see inset photo), and it's easily bendable. So in addition to cleaning your espresso machine, this brush is also useful for other odd jobs.