Tai Chi is on the surface a straightforward study of how to use the body in an integrated way to perform movements which are martial in origin and intent.
While our habituated use of strength and weight will involve a few major muscles tensing for structural rigidity while a few more move a limb, Tai Chi's approach to movement is to persuade almost all of the muscles to contribute equally to every change of position. We couple this with the in/out cycle of our breathing, allowing the flux between activity and rest to reveal the inbuilt rhythm of the sequence of movements, or Form.
In the course of working like this, other effects emerge. We realise we can 'root' ourselves whilst developing a relaxed lightness that doesn't cause us to as it were float away. The many circulations in the body flow more readily, and we discover reducible or removeable blockages in these flows. Our increasing body/mind conscious awareness may well extend to engaging physiologically and energetically with the movements of others in partner work such as Tui Shao ( 'pushing' hands).
Tai Chi has in recent decades moved all around the world and been adapted by teachers with many personal preferences. Tai Chi for health, longevity, meditation, competition, as well as sparring or self defense are on offer everywhere. The Art offers, or should offer, all these things and more, so should not be narrowed down to a single function or idea. Thus it grows with time, like any genuine art.
It is described as a Taoist* art, meaning it follows precepts central to Taoist philosophy, such as following a natural course without resistance, adapting completely to circumstance, and avoiding mindless habit, while choosing direct perception and observation over fixed belief.
Let's just add here that there is a vast amount more to say about Tai Chi, but it's so much better simply to get on and practise!
*Taoism does not refer here to the Taoist religion, but to the profound philosophy of body and mind from which the religion borrows the name.