I don’t know what’s good about tea (I don’t drink any), but I am somewhat familiar with how antioxidants work. In a nutshell (in a super-super nutshell – it’s quite a bit more involved than this), antioxidants resist the oxidation of molecular compounds. Oxidation (again, in a nutshell) means losing electrons. In the process of oxidizing a compound, you can create what’s called a free radical, which is an atom or molecule with an unpaired electron. Because of this unpaired electron, it is extremely reactive – this can be good, but it can also be very bad if, say, it were to affect DNA and cause a mutation.
So that’s the gist of it. Don’t get the impression though that free radicals are all bad and antioxidants are all good. Everything is necessary for certain things.