Ableton Live allows you to bus audio out from multiple tracks (say vocals, drums, lead synth, and pads) to a single track that has FX on it (for example reverb). This bussed track is only for FX. You can independently dial in how much volume from each track will be sent to this FX track. Hence the term send and return. You would think that there are 2 stages, but there is really only 1 FX track in this scenario. The FX track then routes its audio to the master (by default, but you can change the routing to a track separate from the master.)
Note, return tracks act in parallel, meaning that even when a track’s set to send 100% to the return track, the original audio is still going to the master, meaning 100% wet is really 50% wet, 50% dry.
There are also Pre/Post buttons for sends. These buttons are located on the master track, above the master fader in session view. Post is the default. When set to Pre, the input of the return track will ignore where the fader is set on the source track that is being sent to the return track. This means that if the source track’s fader is pulled all the way down, but it still is set to send some amount of its audio to the return track, you will still hear the return track fx. When set to Post, if the source track’s fader is pulled all the way down, you won’t hear anything on the return track fx.