We don’t know. Science tells us it definitely doesn’t change the physical properties of our muscles. They are a way to tough for that.
There is an idea that the roller stimulates our nerves and sends a message to the brain that, ‘this area is OK, just tender’, then the brain ‘down regulates’ the signals to the muscles and the muscles relax. Much like a massage. You definitely feel nicer and lighter afterward.
There may also be some fluid effects like helping move our lymphatic fluid around which is our bodies garbage removal process. This fluid kills off all the microdamage and lays down the good stuff after exercise. Rolling and massage may aid this process.