I’ve heard Chi summarised in technical medical terms as “stuff”.
Canonically speaking, Chi is considered to be “life force” that essentially comes from the breath, is stored in one or several “dantians” or energy points in the body, the most prominent one being in the underbelly, as @LostInParadise said; two others are in the chest behind the sternum and in the head behind the brow, between the eyes. It’s released again from there to flow through pathways (meridians) in the body.
Despite my respect and fascination for ancient Chinese culture, I’d have to classify Chi as on par with phlogiston and aether as an ancient but discredited explanation of how stuff works, and with astrology and homoeopathy for still being clung to in spite of the above.
To the credit of the notion, like @LostInParadise noted, it’s useful as a visualisation in martial arts. I can confirm this from four years of Tai Chi practise. But in that regard it is on par with the notions that your cranium is hanging from a golden string and your feet are rooted in the ground; just a visualisation.