Travell and Simons (1992) defined a trigger point as a highly irritable localised spot of exquisite tenderness in a nodule within a palpable taut band of muscle tissue. In short: knots in the muscles (overcontracted sarcomeres, the smallest part of the muscle where contraction takes place) that are very sensitive to touch and almost always cause referred pain.
So this is a deep muscle massage with the emphasis on trigger points.