Normal Stretching is not always the answer to tight muscles. You can have a muscle that is tight but lengthened. The upper back has this a lot, where the muscle between the shoulders are stretched but tight. Traditional stretching cause increases in muscle length and not all tight tissue needs lengthened and stretching it can actually increase your chances of injury.
Self-myofascial release on the foam roller breakdowns soft tissue adhesions and scar tissue in fascia. Fascia is a connective tissue that connects all different parts of the body. Fascia surrounds muscles and abnormal adhesions can lead to muscles not working properly and pain.
A foam roller works partially due to the principle known autogenic inhibition. There are nerve receptors in the fascia and one of the things they detect changes in tension in the muscle. Stimulating these by pressure causes the muscle to relax without lengthening the muscle. It also breaks down any abnormal adhesions to let the muscle work correctly