Send and return tracks are a method of incorporating a parallel channel strip of a processed signal while retaining the dry, unprocessed signal on the first channel strip. This allows you to blend and mix the two signals together to taste.
This method of processing is called parallel processing due to routing of signal flow on independent channel strips.
Consoles are complete with master axillary send and returns that you may use for common parallel processing methods such as a popular reverb or drum compression setting. Based on a consoles functionality, more return tracks may be created with a secondary channel strip via buss routing. In all cases, return tracks are used to retrieve the processed signal from an effect and into the mix via the return track faders.
Often, you will find both stereo and mono return tracks. This is due to the nature of time-based effects. In order to perceive their effect, a stereo return track must be used.
Return tracks are very much like standard channels except a signal must be routed to them, they do not have an input for recording microphones.