Improvisation is when you make up something on the spot without planning it beforehand. For example, if you're playing with your toys and you suddenly decide to make up a story about them, that's improvisation.
In music, improvisation is when musicians play without a set plan or sheet music. They have to make up the music as they go along. It's like making up a story with your toys, but using sounds instead.
This can be really fun because it means that every time you play, you might come up with something completely different! It also takes a lot of practice to get good at, because you have to be able to come up with things quickly and in the moment.
Overall, improvisation is all about using your imagination and creativity to make up something new and exciting!