This is one of the tougher aspects to grasp and it also involves bussing the signal along a path . I like to think of bussing in a physical form and that is what helped me learn how to understand sends and returns. Any buss can be thought of as the School Bus, traveling along a predetermined path, conveying audio signals on an electrical circuit rather than children on a road, the Bus travels on an audio signal path.
On the back of the DJ mixer you will often see a Send and a Return slot in the form of 2 TS ¼ inch jacks ( also known as guitar cable). One set will be for the send and the other for the return. The main use for this feature is to add external effects units onto your mixer in a similar fashion to a guitarists who uses an effects pedal.
The send can be considered an output and will often be assignable using the mixers effects section. This means that you can choose what you want to assign to send as a physical output. For example you can send the master output to the send or you can send an individual channel of audio. The send lead should go into an effects unit’s input allowing the signal to be altered in some form. You can also send the signal to a guitar amp noting that the signal will be a mono (single) source.
The return jack is an input and as you may have assumed, is used to return the signal that has been sent from the send. You will plug the output of your external effects processor back to the return so that you can get the effected signal back into your mixer.
You can take the audio signal using the send and connect it to an external device such as a effects (FX) unit and then return the output of the external device into the return in the mixer. Most of the time, for this process to work properly, you will have to engage the SEND/return function on the mixer usually in the form of a rotary switch. The mixer will usually allow you to route any signal to the aux send/return, from individual channels to the entire master output. This is a powerful tool because external devices can range from Guitar pedals to external FX units to looping stations.