The noise floor is the level of background noise from equipment and the environment that is present even when there is no signal being recorded or played back.
The basic idea
If the sound you want is too quiet, the gap between it and the noise floor shrinks and the noise becomes easy to notice.
What to check when you use it
Listen to a silent passage before recording, and review your cables, power, mic position, and input gain.
The noise floor is the background reference, while the S/N ratio expresses the ratio against the wanted signal.
