The mute is a device that you attach to a music instrument, to change the instrument's sound quality, and/or reduce its volume. It can be either attached for the whole duration of the performance, or be added/removed during the performance. Those can also be used for practice purposes, with the intent of reducing the volume of the sound produced by the instrument.
Select a note then click on the mute button to add a mute to the instrument, or click on unmute to remove the mute. You can find those buttons in the articulations toolbar.
Currently, all our HQ brass instruments can have their sound affected by the mute on Flat.