Qigong broken down in translations roughly means "Qi" being "energy-breath" and "Gong" being "work/cultivation", combined it means Energy Work/ Breath Cultivation. It is a system deriving from China, which then spread into Far East Asian (Korea, Japan). Qigong is an umbrella term which encompasses many schools of thought- be it philosophical, regional, and even by specific-purpose (for more information on "Types of Qigong", click here). However, they are all generally based around core systemic concepts aligned to Traditional Chinese Medicine. These concepts are Meridian and Dantian Theory.
Meridians refer to channels throughout the body where Qi flows certain directions. These channels are corresponded to bodily functions and emotional/mental states. Dantians are "Elixer Fields" where the Qi is gathered and stored. There are 12 Meridians, 8 Extra-Ordinary Meridians and 3 Dantians.
Common Qigong Sets:
Baduanjin (8 Set Brocade), Yijinjing (Muscle/Tendon Changing Classic), Wu Qin Xi (5 Animals Play), Tai Chi/Taijichuan (this is also a martial art)