It is a form of limitor/compressor.
Some times called a Brickwall limiter.
The Waves L1 Ultramaximizer is a software example.
I use it to final mix and also master tracks.
The Aphex Dominator is a very nice hardware example.
[ We recomended that to Tina Turner for her big album tour
and it prevented her voice, and the lead players solos,
from breaking up the system in one of the freq bands of their 5 way PA.
It saved them adding something like $65,000 more to the PA for only $1,200,
She made us dinner backstage, (veggies) and sent us a nice card and bottle of wine later.]
Basically it sets an absolute output level,
say radio station maximum signal power,
or in a studio or pa system digital clipping or distortion point
and either brings the level up to close to the max,
and then "brick wall's" as in run into a brickwall.
But the Dominator has a look before feature that senses
a peak coming and gently and invisably as possible prevents
it from passing the max output.
The Waves L1 lets you bring your mix level up and set a ratio
of hard limiting over threshold so you can not get digital clipping
from your peaks, but get as hot a track as possible..
I like a brickwall limiter for small clubs with many different performers
and and underpowered system.
Let-em be stupid as stumps with the volume
they ain't NEVAAA gonna blow up a systen that cain't distort!!!!

DLD, Chili farmer. Plus bananas and papaya too.
Real happiness has no strings attached.
But pedal steels have many!