The Status buttons show the current session state:

Solo

Blinks when one or more tracks are being soloed, see Muting and Soloing. Clicking this button disables any active explicit and implicit solo on all tracks and busses. Clicking this button desactivates the solo on every track/bus.

Audition

Blinks when some audio is auditionned, e.g. by using the import dialog, or using the Audition context menu in the Regions List. Clicking this button stops the auditionning.

Feedback

Blinks when Ardour detects a feedback loop, which happens when the output of an audio signal chain is plugged back to its input. This is probably not wanted and can be dangerous for the hardware and the listener.

The Mode Selector allows switching between Editor, Mixer and the Preferences window. If a window is detached, the corresponding button is lit in blue. Clicking the botton switches the detached window visibility.

The global meter shows the levels of the master's output. Its the same meter that sits in the Master's Mixer strip, and also shows a peak indicator, that turns red when any level exceeds 0dB. It can be reset by a Left click.