This lists the tracks and busses that are present in the session. The list order reflects the order in the editor, and track or bus names can be dragged-and-dropped in the editor list to re-order them in the editor. The columns in the list represent the following:

V	whether the track or bus is visible; they can be hidden, in which case they will still play, but just not be visible in the editor; this can be useful for keeping the display uncluttered.
A	whether the track or bus is active; unactive tracks will not play, and will not consume any CPU.
I	for MIDI tracks, whether the MIDI input is enabled; this dictates whether MIDI data from the track's inputs ports will be passed through the track.
R	whether the track is record-enabled.
RS	whether the track is record safe; a record safe track cannot be armed for recording, to protect against a mistake.
M	whether the track is muted.
S	track solo state.
SI	track solo-isolated state.
SS	solo safe state.

Each icon in these columns can be clicked to toggle the track/bus state, which is a very fast way to set multiple tracks/busses state at once.

As with the region list, hovering the mouse pointer over a column heading shows a tool-tip which can be handy to remember what the columns are for.