Clipping and headroom
Clipping cuts the top off a waveform that exceeds the output. Headroom is the gap under that ceiling. The master limiter can catch the peaks, and it adds delay while it is on.
Clipping is a waveform with the top cut flat. The signal went louder than the output can hold. On one drum hit that can be a hard edge you wanted. Across a whole mix it sounds broken, and turning the master down afterwards only makes the flat top quieter.
Headroom is the gap between the loudest peak and that ceiling. The Master page suggests keeping peaks below about −3 dB, and leaving 3 to 6 dB, so a later EQ boost or limiter still has room. If you need the track louder while you listen, turn up the speakers. Pushing the master fader past 0 dB spends the gap inside the project.
The master channel has a limiter for peaks that would otherwise clip. It is off by default, because playback is noticeably late while it is on. See latency. Evening out a performance is still the job of compression on the channels. A limiter is that same idea pushed to a very high ratio, parked at the ceiling.