"Your muscles are actually bundles of muscle fibers that range in size and quantity depending on how those muscles or body parts are used"
There is a lot of information that I could deliver about muscles. There are textbooks upon textbooks about what muscles are made of, the different kinds of muscles, how each different kind is used differently, and so much more. You don't want to read about that here. If you want lots of intricate details, call upon the almighty Google. Basically though, massage deals primarily with muscles, and muscle pain is one of the biggest things people seek out massage to help, so I would be remiss if I didn't at least touch on some of the basics.
If you grab hold of a muscle on your body, such as your upper arm or thigh, it may feel like one giant muscle. In truth, your thigh alone is made up of four major muscles, and a few additional smaller ones. And even if you can find and isolate just one of the muscles, it still may feel like a solid mass but it isn't. Your muscles are actually made up of individual fibers, all bunched together. Imagine you have a bunch of rubber bands - I mean a LOT of rubber bands. Stretch them out, line them all up, then gather the tops and bottoms together, and that is pretty much how your muscles look.
So when my clients ask me "what is that knot I feel in my muscle?" I explain it like this: All those fibers - they may on occasion get tangled up, torn, or not working in the perfect way. Those fibers, if they are all going in the same direction, allow stuff to move in, around and through them - stuff like blood, fluid, lymph, toxins on their way out of the system, etc. So if the fibers get tangled, first of all the fibers aren't going to feel or function right, and secondly, that fluid stuff isn't going to move through them, and can get "backed up" and "stuck" which is why sometimes those "knots" feel warm or like they are burning. (There is actually much more to this, but again - trying to keep it simple)
Massage helps by working to untangle those muscle fibers, and push all the backup through so that the fibers in that area can remain (or return to) soft, stretchy and pliable. Sometimes things untangle easily, but the longer or worse they are tangled, the harder it is to get untangled. Also if the muscle is injured, and those fibers repair themselves with scar tissue, massage may have to work harder or slightly differently to soften that stuff up.