Muscle memory is like when you learn to ride a bike or play an instrument. Your muscles learn how to do it so well that you don't have to think about it anymore. It becomes automatic, like brushing your teeth or tying your shoes.
Our muscles have something called "motor neurons" which are like little messengers that tell our muscles what to do. When we repeat a task over and over again, our motor neurons get really good at sending the right messages to our muscles to do that task.
It's kind of like how you memorize spelling words. The first few times you have to think really hard about how to spell the word, but after you practice a lot it becomes easy and you don't have to think so hard anymore. That's because your brain has memorized how to spell the word.
Similarly, when you repeat a physical activity over and over again, your muscles remember how to do it without you having to think about it. That's muscle memory!