This Studio Ultimate effect helps compensate for a common problem in recording audio for video productions: the imbalance in the recorded volume of different elements in the original audio. For instance, your commentary as you shoot the video may be recorded at such a high level that it overwhelms other sounds at the location.
The trick in using the Leveler is to find a target volume somewhere between that of the loud and soft audio in the original clip. Below that volume, Leveler acts as an expander, increasing the original level by a fixed ratio. Above the target volume, Leveler acts as a compressor, reducing the original level. With careful adjustment of the parameters, the internal balance of the audio can be significantly improved.