A “limiter” is commonly used to limit the level of an audio signal to a specific “threshold level”.
Limiters are used as a safeguard against “signal peaking” (also known as “clipping”).
Another way to understand the function of a limiter: it suppresses the occasional signal peaks that would be too loud or distorted. For example, if a threshold level has been set to -4db, the limiter will suppress or push down anything that exceeds that threshold value and will cap the signal level at -4db.
In this way, the limiter prevents any additional gain above the threshold level. In the mastering stage of music production, a limiter is used to eliminate any unwanted headroom.