Muted brass is when brass instruments have a mute placed in them. A mute is a device used to reduce the volume and alter the tone of an instrument. It is particularly important for brass instruments, which can be extremely loud!
The most common type of brass mute is cone-shaped and fits into the bell of the instrument. Mutes can help brass instruments blend in more and are often used in Big Band and classical music, as well as jazz ensembles.