Everyone knows the "five" senses, but in reality, humans have a lot more than five.

One of those senses is called "proprioception," the ability to know how your body is positioned (and moving) without other inputs (like sight). Oversimplified, proprioception is essentially the brain's communication with the nerve endings that connect to your muscles (e.g., motor neurons).

But your senses are not fully developed at birth. Your brain has to work out the signals it sends and receives and build those up into a structured system. "When I receive these signals, my hand is here. Those signals, my hand is there. These other signals..." etc... The process of building that communication between your brain and your motor neurons is building "muscle memory."

Something that makes it a bit clearer is to think of hitting a growth spurt. Your sense of position, the interaction between your brain and body, is unchanged, but your body is now taller (or stronger, or missing a limb, or supplemented with a mechanical prosthetic...) so your brain's trained response doesn't match what you actually want your body to do, and you have to re-learn and adapt your brain's connections to your new body structure.

And to do that, you have to repeat a motion again and again and process the results, refining your movement to match your intent. Eventually, after enough repetitions, your movement will be more consistent. Perhaps even habitual. And that's muscle memory.