The concept of Qi (Sometimes written as Chi or Ch'i) comes from Chinese traditional medicine and its translation has been approximated to 'life-force', air, breath or even energy-flow. It is understood by practitioners as a vital force or energy that is required for life and if unbalanced or unable to flow properly, causes illness or poor health in a living entity. Similar concepts are found across East Asian cultures and there are references for it going back to the 5thC BCE. It was a concept embedded in Chinese philosophy and used to help understand the world around them.
Some non health practices use it to, Feng Shui for instance, is based on calculating the balance of Qi, and the retention or dissipation of Qi. It does this by arranging the layout of a living space, which is believed to affect the health, wealth, energy level, luck, and many other aspects of the occupants. Some martial arts use the concept of channelling or focusing Qi to explain seemingly miraculous powers that proponents can get when performing certain feats.
Many more modern alt-med modalities try to use Qi to justify or explain their treatments. Reiki for instance is a classic example (and will be covered in another fact) but any use of Qi (or Chi), especially when trying to sell you something, is a pretty sure way to determine that it is bullshit. There are countless articles showing you '10 ways to Balance/connect/increase/harmonise etc your Chi'. They are best avoided.