"Tai" in Chinese means "Adjective + est", and "ji" means "the utmost". "Tai chi" is the English speaking sound while "Tai ji" is the pin yin of the two Chinese characters.
"Tai ji" is a philosophical concept in its literal sense. And later, our ancestors named the martial art they created as "tai chi chuan" which in Chinese pin yin should be "tai ji quan". "quan" means "fist or martial art".
It seems if we truly want to understand tai chi chuan, we have to know a bit of its philosophy hidden behind. And speaking of "tai ji" in its literal sense, we have to mention Taoism.
The main idea of Taoism is that people should live a simple, natural, and honest life. Our ancestors believe that if you fail, it's because you are not behaving naturally, and you are not honest to yourself. We will discuss more on this point in the future topics.
So now you may understand why most of the tai chi chuan instructors are keeping telling you "be natural".
I'm always thinking that a martial art is a martial art is not because of its "martial" part, but of its "art" part. And tai chi chuan has a powerful idea as its soul, that's probably why we are so crazy about it.