In cupping a glas cup is placed on your skin and by reducing the air in the cup, your skin gets pulled into the cup. This feels similar to massage.
From a Chinese point of view, cupping draws stagnation (sha) that is deep into the body to the surface where the body is able to deal with it and relieve it. If sha comes to the surface the skin gets purple/blue. It looks a bit like a bruise but it does not hurt. This discolorisation takes about a week to disappear.
Cupping was not only known in China. It was for example widely used in Europe. The mother of the house would treat her children/husband at the start of the cold season to prevent them getting a cold. It is frequently used for asthma and bronchitis, and for back and neck pain. But it can do much more. Basically cupping can be applied to any bodypart that is big enough and flat enough to hold a cup.