In this photo, the redness on my back from a massage was not a painful skin infection. It looks bad, but I felt better because it relieved my tightness. This reaction is called hyperemia which is the presence of an increased amount of blood flow to an area.
Athletic training can produce microscopic damage to the muscle and fascial* tissue. Blood then moves less efficiently in these damaged areas. Old injuries, scar tissue, and muscle tension decrease blood flow to muscles and slow down the removal of cellular waste. Massage helps muscles recover, whether from training or tension, by increasing blood and nutrients circulating throughout the body. Hyperemia is created by the pressure felt in front of the massage stroke along with suction created behind the stroke. This brings more blood into the area. Microscopic tears are healed and muscle tension eased.
*For information about fascia, check out this article: https://breakingmuscle.com/learn/the-top-5-ways-fascia-matters-to-athletes