A leveler performs an audio process similar to compression, which is used to reduce the dynamic range of a signal, so that the quietest portion of the signal is loud enough to hear and the loudest portion is not too loud. A leveler is different from a compressor in that the ratio and threshold are controlled with a single control. Levelers work especially well with vocals, as there are huge dynamic differences in the human voice and levelers work in such a way as to sound very natural, letting the character of the sound change with the different levels but still maintaining a predictable and usable dynamic range.
References
Leveler Wikipedia(Text) CC BY-SA