A leveler is a device which uses an audio gain circuit to increase or decrease the level of audio passing through it based on parameters set by the user. The purpose is not specifically to reduce the dynamic range of a signal like in a compressor or limiter (though that is often what happens), but to simply have an audio signal stay at roughly the same volume for an extended period of time. Most levelers are not sensitive to peaks of a short duration, but affect very gradual changes to audio level as input levels gradually change over time. Sometimes levelers are referred to as automatic gain controls (AGC). A high quality leveler will work far better and more transparently than the typical AGC control you might have used that was built into an audio or video tape recorder.