When I have been asked this question, I think of it like a two minute elevator speech where I encapsulate my life in two minutes. I Describe my background (i.e., where I am from, siblings, parents, wife, our cat, my hobbies), highlight major life pursuits (i.e., 20 year military vet, finishing my PhD, one or two significant research highlights), discuss my community service passions, finally, I describe one or two of my future goals. You shouldn't stick to a script that rehashes all the stuff in your secondary. I try to make my "tell me about yourself" answer as unique as possible (i.e., stuff not in my applications).