Musical improvisation means making up music while you are playing or singing. It's kind of like when you and your friends make up a story together as you go along. You don't need to follow any rules or written music, but you just play or sing whatever sounds right to you.
It's like a game, but the instrument or your voice is your toy, and you create new sounds and melodies with it. Sometimes, you start with a melody or a few notes that sound good, and then you keep adding on to it until it becomes a whole song. Other times, you might just start playing or singing whatever comes to your mind, without much of a plan ahead of time.
Improvisation happens in many different types of music, like jazz or rock, but it can also just be something you do on your own, like humming a tune in the shower. It's a really fun way to express yourself and make music that comes straight from your heart and mind!