Each track has a mute switch, which determines whether the track is enabled or disabled. Muting applies to all track types, and can be modulated via mute modulation. Muting a note track only prevents it from starting new notes; any notes that were already in progress when the track was muted continue until they finish their durations.
Muting is done via the track mute buttons, which are located immediately to the right of the vertical divider that separates the two panes of the track view. If a track is muted, its mute button is red, otherwise it's green. To toggle the mute state of a single track (from unmuted to muted or vice versa), simply left-click the track's mute button. To toggle the mute states of multiple tracks, first select them, and then left-click anywhere within the mute buttons area, or use the Track/Mute command.
To unconditionally mute the selected tracks, regardless of their current mute state, left-click anywhere within the mute buttons area while pressing the Ctrl key.