Sends and Returns
When looking at outboard gear, you may notice a send return option. This gives you the ability to insert an effect into the audio chain. This is called an insert.
For instance, if you have a preamp with ad converters, it will often have sends and returns. The way this works is the signal gets boosted in the preamp, and then is routed to the send. You may then use this send signal to route it into a hardware compressor or reverb, or any effect you like. You would then patch the output of the effect to the return. The ad convertor will take its signal from the return.
This can be used in a few different ways. If you don’t use the return, then the send acts as a duplicate of the signal. So the signal will still go to the ad convertor, but will also be sent out the send. This could be useful for live monitoring, or for duplicating the signal.
If you use only the return, then you are bypassing the preamp and using it as a line input to the ad convertor. As soon as you plug something into the return line, the ad convertor receives its signal from this input, and not from the preamp. This is convenient if you have a hardware instrument like electronic drums or piano, you would use the line output from the instrument plugged straight into the return.
So just to recap how sends and returns work, the effect is called an insert, and the signal is being sent to the insert, and returned from the insert.