The status bar is an informative bar at the top of the window. Right clicking anywhere on the Status Bar allows to choose which of this information is displayed, through a checkbox menu, among:

Path to Session:	the folder where the session files are
Snapshot Name and Modified Indicator:	the name of the snapshot being worked on. If the session has been modified since last save, the name is prefixed with an asterisk (*).
Active Peak-file work:	(only shows up while creating peaks) displays the number of peak files left to create
File Format:	the file format used in the session, including when recording
Timecode Format:	is the timecode, i.e. the number of frames per second used by the session (for videos)
Audio:	gives the sample rate used in the session, and the latency computed from the buffer size
Disk Space:	reports the remaining hard disk space as the time that can be recorded with the current session setting
DSP:	for Digital Sound Processing, shows how much of the CPU is used by Ardour and its plugins
Wall Clock	showing the system time (especially useful in full screen mode)
Log button	that indicates if Ardour has encountered any warning or error.

Some of these indicators also double as a shortcut to a relevant action, triggered by double clicking the info field:

Path to Session / Snapshot Name and Modified Indicator	opens the session path in the file explorer
File Format:	opens the Media section of the Session Properties
Timecode Format:	opens the Media section of the Session Properties
Audio:	opens the Audio MIDI Setup window.
Log button	The log button turns yellow when a warning is shown, and red when an error occurs. Clicking the log button gives access to the log.