It's a jungle
Not to mention in how many ways it has been interpreted outside of the Far East traditions.
I have studied the concept some, also comparing it with similar concepts in other cultures. Such a term, used for some or other kind of life force, also related to air, breathing, wind, you find in many traditions in the world. Prana in India, pneuma i Greece, ruah in Semitic traditions, and so forth.
I guess that at the bottom of it, they deal with the old mysteries of what is the difference between being alive and being dead (and what happens to that which made someone alive, when he has died), what makes man and animals able to move, and what makes them will to move, et cetera.
has an interesting etymology in its kanji, consisting of the sign for vapor/fume/air and that for rice. Boiling rice. To anyone in China and Japan it's a very relevant symbol of life energy. You gotta eat.
You could also call it a symbol for that in the air, which you need to stay alive.
That would be oxygen, of course. It's just a few hundred years, since it was discovered in Europe. Before that discovery, it would be most reasonable to imagine some life energy hidden in the air - actually, it would be quite accurate, although not expressed in scientific terms.
Sorry, I digress...
is in the core of aikido, and maybe one's understanding of ki develops just like aikido as a whole does: by training.