Athletes, celebrities, and people interested in "natural health" may sometimes receive cupping treatments, in which a warm glass cup is placed on the skin, usually around the back or neck. As the cup cools, a vacuum is created within the cup drawing the skin up into it. This creates a very distinctive, circular, red welt. Seeing these marks almost guarantees that the person received a cupping treatment. You could reasonably expect that the person with the cupping marks is either an athlete or engages in some kind of massage or similar "self-care" service, possibly that they think this process will help draw toxins from their body.