Heat therapy is the application of heat to specific parts of the body. Moxibustion can only be used for this purpose, but I generally just use infrared heat lamps and a electric blanket on the couch.
The use of heat in Chinese medicine is very important. Chinese medicine never uses cold therapy, as it's believed cold works against the body. Qi (energy) and blood move better when the body is warmer. Blood is a liquid too so is affected by the cold. People fall ill in winter because their immune system, which is housed in the blood moves slower allowing viruses to get the upper hand.
Unlike physiotherapists or chiropractors, Chinese medicine never recommends the application of cold packs to areas of the body with inflammation and pain. Cold in Chinese medicine causes stagnation which slows the body down from working properly.
Qi (energy) is also heat, so more heat can aid Qi's flow better. Heat allows the body to work more efficently. For example, when you fall ill, you get a raised body temperature so the immune system in the blood can work more quickly and efficiently and attack any pathogens invading the body.