Ki just is people's life energy, a concept is the idea of said person existing.
What’s the secret of winning in battle?
When it comes to battle, the most important thing is ki size, and its control. Of course,
“ki” also includes such spiritual power as energy/vigor [genki] and bravery [yuuki], and being in
one’s right mind [shouki; could also be translated as "
true character"] (note 11). There’s a limit to physical strength, no matter how much you toughen it up, and the only way to overcome that it is with “ki”. I think that it was through
turning ki into formidable power that Goku drew closer to being the strongest warrior in the universe.
Note 11: Genki and yuuki, shouki
The word
“energy” [genki; also “health”, “vigor”, etc] is well known, but if you had an illness [byouki], would you not have energy? That’s difficult to answer. There are some people who have a healthy body but no energy, but there are also some people who are full of energy even when ill. What’s more, even with people who have about the same amount of power, there are some who, when attacked by Piccolo Daimao for instance, will stand firm and tough it out, while others will run away trembling in fear. If you think this way, you might understand how different kinds of “ki” are very important in battle. In the world of Chinese kenpo disciplines such as Tai Chi,
much importance is placed on the training and control of ki.
Although in “DragonBall” the wizard Babidi had invented machines that measured and extracted “ki”, modern science is unable to measure “ki” or store it anywhere. However, there are times when people support you, and
give you energy [genki] and
courage [yuuki]. Though
it can’t be seen, what is present then is
“ki”.
Akira Toriyama SEG interview
Mind, Health, Courage, Vigor/Energy, Power, Spirit, Character and Bravery are all fundamental attributes of Ki to say its just "people's life energy" seems to cheapen what Toriyama says here. This concept of ki is even compared to Kenpo and specifically Tai Chi which is a philosophical concept.