Saturation is a distortion effect known from analog tape recordings.
Back in the days when you were recording to tape you had to be careful with the level - too low would introduce noise in the tape recording, too high and the tape would reach a saturation point where no more signal could be recorded. Unlike digital clipping an analog tape would break down in a non-linear way, which actually has quit a nice sound to it.
Saturation Knob is a free plugin from Softube. It is very easy to use: Just turn the big knob to fatten the bass line or add harmonics to the vocals.