Okay kiddo, so you know how when you're playing with your toys in your room, other people in the house can still hear you even if they're not in the same room?
Well, that's kind of like what happens with sound in a recording studio. When musicians are playing their instruments and singing, the sounds they make can "spill" over into the microphones that are picking up other sounds.
Imagine you're painting a picture with different colors. If you accidentally get some of one color on a different part of the picture, that's kind of like audio spill in a recording. It's when sounds that are supposed to be separate and distinct get mixed together, and it can mess up the quality of the recording.
That's why studios use special equipment and techniques to try to minimize audio spill, so that each sound can be captured as cleanly as possible. It's like using a special paintbrush to make sure only the colors you want go where you want them to go.