signal levels are usually indicated on audio equipment by means of some
kind of meter.
The "0" marking on these meters shows the level at which the device
will provide the best audio performance, with minimal noise, no distortion,
and some headroom to spare.
is generally a good idea to run signals as high or "hot" as possible without
distortion. This will result in the highest signal-to-noise
ratio and quieter, "cleaner" audio. Even if a sound needs to be at a low
level in a mix, it's better to record it hot and attenuate it to the appropriate
level when recording the final mix. This will result in the inherent noise
of the recording media, processing, and mixing components also being attenuated
with a resulting quieter recording.