Saturation is what happens when audio is turned up too much – so much that the next device in the chain can’t handle it. The result is that the loudest parts of the sound are distorted and the quieter parts of the sound are left unchanged. This dynamic behaviour is similar to a compressor, except it’s much more extreme. Normally audio engineers try to avoid saturation and distortion as much as possible, but in the mix it can be used as a creative effect. The way saturation affects sound depends on the nature of the sound itself.
For sounds with strong transients (such as drums and percussion, or other ‘peaky’ sounds), saturation reduces the level of the transient peaks by distorting them. Because the peaks are very short, however, the distortion is sometimes not very noticeable. Instead of sounding like distortion, it sounds like the peaks have become noisier and dirtier. For some kinds of music, this is desirable. The power and impact of the sound is enhanced (even though fidelity suffers).
For sounds with a more steady level (such as organs or strings), saturation is often more noticeable because the sound is constantly being saturated. For these sounds, saturation usually adds brightness and harshness. Used tastefully, this can make a sound more exciting or aggressive. Too much saturation, however, will make the sound lo-fi or distorted.
In a dense mix, it’s usually possible to get away with more saturation because the noise created by the saturation blends in with the background of the mix.