I think the answer to that is short. And perhaps it’s simple or very difficult, depending upon who you are and how you feel. Besides suggesting that there is a tomorrow, that there is challenge and romance in the world, besides suggesting that it’s not all over, STAR TREK said something else, and I wonder if you felt it too, because we meant to say it. It goes rather like this: "You human biped thing called man, you strange creature, still you’re awkward and often illogical, you’re weak, vain, but damn it, you’re also gorgeous.” And we were saying, “Hey, we love you.” And I think that’s what it was about, really: “We love you.

From a 1975 issue of “Star Trek Prospers” (where its source is not credited), Gene Roddenberry answers the question “What is Star Trek about?”  (quoted on Fanlore, the fan run wiki about media fandom)