When we talk about muscle tomorrow, we’ll be looking at a structure called a sarcomere, which is the basic contractile unit (for lack of a better phrase) of muscle. It’s the actual thing that shortens when you want to move a muscle, and the more you learn about it, the more amazing it is.
We will talk about the composition and appearance of the sarcomere, and we’ll also talk a little about how it works (because otherwise it would be really boring). You’ll learn all the details in physiology, so for now, you just need an overview. Here’s the best video I’ve found for that purpose.