Ok kiddo, you know how when you listen to music, sometimes different songs have different beats and rhythms? Well, a DJ mix is when someone takes a bunch of different songs and puts them together in a way that makes them all flow really nicely together.
Basically, it's like you're listening to one big song made up of lots of little songs! The DJ uses special equipment to do this, like turntables or a computer program. They can also add in special sound effects and change up the tempo or volume of the songs to make it even cooler.
It's kind of like a musical puzzle - the DJ has to figure out which songs will work well together and how to arrange them in the best way. And when they're all done, you can dance and have fun to the mix they made! Pretty cool, huh?