Okay kiddo, you know how when you're playing with your toys, sometimes you like to play fast and sometimes you like to play slow? The same thing can happen in music, but instead of toys, it's the notes that musicians play.
Tempo rubato is a special way of playing music where the musician can change the speed of the notes. It's like they're going on a little adventure with the music and taking their time in some parts and going fast in others.
It's kind of like when you're running and you slow down for a bit to catch your breath, and then you speed up again. Musicians can do this with their playing to add emotion and feeling to the music.
So, even though the music might have a certain tempo or speed written down, the musician can change it a bit to give the music more personality. It's like they're coloring outside the lines a little bit!