Deep tissue massage involves manipulation of the soft tissues to reduce muscle tension, dysfunction, discomfort, and pain. There are many layers to the body and different techniques to work on the muscle fibers, fascia, and attachments. 

Studies for massage therapy have shown that they help with lower back pain:
Deep Tissue Massage and Nonsteroidal Anti-Inflammatory Drugs for Low Back Pain: A Prospective Randomized Trial (hindawi.com) Retrieved 14 Jan 2022