A processor is something which is applied to the whole signal, different from an effect as effects are something which are applied to part of the signal only. Processors include things like EQ, compression, limiters and gates; I have chosen to explore limiters. Limiters are similar to compressors, being that they are an amplifier that limits the upper dynamic range of a signal to a specific threshold. Unlike a compressor however, which works gradually to reduce the signal, a limiter prevents virtually any increase in gain at the upper end of the dynamic range. Limiters are used to increase the loudness of a sound by increasing the quietest parts of the audio signal whilst also preventing the peaks of the audio from clipping. It does this by catching the loudest peaks and applying brick wall compression which prevent the signal from passing beyond the point of clipping at 0dBFS. Limiters are commonly used on vocals and metal drums, such as hats, and are the most common during the mastering process. Brickwall limiters are commonplace in the music industry and work by having high ratios, implementing a delay in the signal. They keep the signal from breaking through a certain ceiling, usually somewhere between -1 and -0.3 dBFS as beyond this you will encounter the problem of digital distortion or clipping.

https://www.izotope.com/en/learn/an-introduction-to-limiters-and-how-to-use-them.html