The Mounted Volume trigger executes when a volume (disk) is mounted or unmounted.
It can be a specific volume or any volume. To specify the volume, you can choose to do it with the exact name of the volume, or a value with a name containing a specific string or a name matching (regular expression) a specific string.
When the macro executes, its %TriggerValue% token will hold the name of the volume (technically, the last path component, which will sometimes be slightly different to ensure uniqueness by the system).
You could use this trigger to perform actions when you connect a backup disk, or automatically sync files with a disk when it is mounted.