The status bar is an informative bar at the top of the window, showing:

File:

the file format used in the session, including when recording

TC:

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

Buffers:

decribe how much data is buffered, see below

DSP:

for Digital Sound Processing, shows how much of the CPU is used by Ardour and its plugins

PkBld:

(only shows up while creating peaks) displays the number of peak files left to create

X:

shows the number of xruns since Ardour's launch, see below

Disk:

reports the remaining hard disk space as the time that can be recorded with the current session setting

Wall Clock

showing the system time (especially usefull in full screen mode)

Log button

that indicates if Ardour has encountered any warning or error.

Right clicking anywhere on the Status Bar allows to choose which of this informations we want displayed, through a checkbox menu.

The buffers are labelled as p for playback and c for capture. If the system is fast enough, these buffers should be 100% full at all times, showing the system has time to precompute all the data before delivering it to the audio system. A buffer constantly under 20% is a sign of an underpowered computer system or of too much processing.

An Xrun (short for buffer over- or under-run) happens when the system has been forced to skip audio frames, e.g. if the latency asked is too short for the computing power of the machine. It usually results in clicks, pops and crackles if it happens while recording.

The log button turns yellow when a warning is shown, and red when an error occurs. Clicking the log button gives acces to the log.