Rash vests or rash guards as they are sometimes known protect you from irritation from your surfboard wax, or worn under your wetsuit protects from any rubbing and gives an extra layer or insulation.Also gives UV protection.
Thermal rash vests are useful in warmer waters where a wetsuit is too warm, they are also ideal for wearing under your wetsuit on colder days where they give an extra layer of insulation. There are two main types of thermal rash vest; the neoprene or glideskin as it is sometimes known or the polyester type, the polyester vest is more comfortable than the neoprene lined type but not quite as warm.