Dynamic range compression, also called DRC (often seen in DVD and car CD player settings) or simply compression reduces the volume of loud sounds or amplifies quiet sounds by narrowing or “compressing” an audio signal’s dynamic range. Compression is commonly used in sound recording and reproduction and broadcasting.[1]

The dedicated electronic hardware unit or audio software used to apply compression is called a compressor. Compressors often have attack and release controls that vary the rate at which compression is applied and smooth the effect.

via Dynamic range compression – Wikipedia, the free encyclopedia.