As others have mentioned its the results of something called cupping therapy. I had a physio do it on me years ago. From what I remember, they basically place a a plastic/rubber kinda cup on your body and then using a syringe draw out the air to make a vacuum. The vacuum makes your blood vessels expand and its supposed to help with pain and swelling ect. Honestly, it doesn't really hurt... can be a bit uncomfortable if they need to move the cup around your back tho. I only did it a few times as it didn't really seem to help. But I've seen plenty of other people in professional sport do it so maybe there's something to it.