In the days when everyone mixed sound using hardware mixing desks and outboard processors like compressors and reverbs, it was easier to see the physical order in which gear was connected. In software, it's much harder to see direct paths between stages of processing and yet it's still vital to understand what order your tools are connected in. That's because levels, effects and EQ can all affect the way the next stage in the chain reacts to signal.
In this video from the course Audio Mistakes 107: 10 Common Signal Flow Mistakes, mixing expert Joe Albano explains why it's important to know about using sends and returns in your mixer. It's a method of applying the same effect to multiple tracks in a project in the most efficient way possible - by routing variable amounts of a track through to the send and then back through the return.
Not only does this save processing power (it comes from a time when you could only use an outboard effect for one thing at a time rather than loading up lots of plug-ins) but it can also help to give a track a more coherent sound, by using the same compressor or reverb on multiple tracks, but using variable amounts. Here, Joe demonstrates how send and return routing works as well as explaining how you can use it to your advantage.