When a muscle is in a contracture (a type of involuntary spasm) for a length of time, it can become tight and ischemic. Ischemia is a state in which blood delivery to muscle fibers is in short supply.
The best way to illustrate is with an example. Think of sweet corn. The ear of corn is wrapped in a relatively healthy-looking husk when in the grocery store - still green, supple and almost moist. You take the husk off and throw it away before you cook the corn.
If you were to take that husk and put it on your counter top for a week or two, what would happen? It would dry up and get brown. It would shrink and shrivel like in the picture above. Muscles don't react quite that badly to a lessened blood supply, but they do suffer. Blood carries important nutrients and oxygen. Plus, blood brings little garbage men to take away wastes. When wastes are left in muscle fibers rather than taken away by a steady stream of blood, they gather to create a noxious environment. Wastes can cause trigger points to form and they can irritate nerves. Therefore, muscles can become tight and painful, just because of waste build-up.
The kneading action of massage brings a flood of blood that hydrates and softens muscle fibers which can restore health and function.If you were to put that dried-up husk into a bowl of fresh water, what would happen? It would likely start to hydrate and expand. It would not come back to life of course, but it would change its shape and size. In a similar way, massage helps muscles to improve by lengthening and broadening fibers. Part of it is the manual manipulation itself, like kneading bread dough. But much of it is the critical action of bringing blood to the area through the pressure which is sort of like bringing gasoline to the engine of your car when you press the accelerator with your foot. Blood floods in which hydrates the fibers, allowing the muscle to relax.