Gua Sha is an ancient technique first used by the Chinese to relieve pain and remove toxins from the body. Gua Sha involves moving an object across the surface of the skin in various locations. In the case of pain, it relieves the body of stagnation (which is considered to be one of the primary sources of pain in TCM) and helps bring toxins to the surface, where they can escape the body.
People who receive acupuncture often enjoy it, saying that it feels like a massage. One thing to note is that Gua Sha does usually cause the skin to appear very red for 1-3 days in the areas that it was applied. However, it is usually a pleasurable experience and virtually painless. People in Asia also often use Gua Sha during the onset of a Cold/Flu to help the body release the illness before it sets in.
Read more about the science behind Gua Sha here: https://www.pacificcollege.edu/news/press-releases/2015/05/05/science-gua-sha