This widget serves two purposes. Firstly, it displays a waveform representing the amplitude changes of a small section of the sound playing on that channel. Secondly, it provides a convenient user interface to control the playback of individual channels.
Each square section of the monitor widget represents a track in the pattern editor, identified by the name displayed in the top left corner. The graph shows approximately 23ms of the final sound playing on that track, after all effects and controls are applied, basically what comes out of the track and is sent to the master sound output.
Using the mouse, you can control the state of these tracks by clicking on the squares representing the tracks.
Left click to mute a track, and then again to unmute it.
Shift+left click to solo a track.
When you solo a track, all other tracks are silenced, only the selected track will be heard. While in solo mode, shift+left clicking on any other track will make that one the solo track, silencing the previous one. Shift+left clicking again on the solo track will turn off solo mode, and return to the previous state, restoring the mute status of any previously muted tracks.