It is dangerous to ask "what is qi?" because people have been debating the answer to that question for over 2,000 years.
If you ask an academician, you will hear something dry and cryptic, like "cosmic energy".
If you ask any serious practitioner of Eastern religion, you will likely be told that it is only something that can be felt, not described.
So what is qi? Your guess is as good as mine.
However, in the context of tea, qi seems to be fairly well-described... though I better not attempt to enumerate the feeling one gets from consuming tea with potent levels of qi, since I've yet to experience it myself. Tea with high qi seems to be associated with a rush of blood to the head and stomach, heightened mental clarity, and lifted spirits. But that's just my vague, nebulous conception.