It’s an act of playing existing recorded music for a live audience. The sole aim is to trigger the subconscious of the audience. This can only be done when the music never stops. In order to create a continuous flow, we need a continuous sound source which is impossible to achieve using a single record player. Thus, arises the need for a minimum of two sound sources (Record Players) and something with which we can switch the music from one sound source to another before the music ends on the former. This something is known as a Mixer. 
A traditional DJ setup would include a minimum of two sound sources (Record Players: Pioneers CDJs) and one Mixer. The sound travels from the Record Players to the Mixer and then is exposed to the audience via the Master Out.
This setup lets us maintain a continuous flow of music. But what about giving the set a direction? For that, we need to listen to the next track while the first is playing. For this, we have a Cue Out in the Mixer which enables us to listen to the next track on our Headphones.
That's it. We'll make it as easy as it sounds for our learners.