This can be quite a frightening question. Somebody is asking about you and you're confronted with such a broad and general opening line. How on Earth do you reply?
When you finally get that all important interview with an agent, you want to be as prepared as you possibly can be. You've probably brought copies of your best headshot along with your resumé. Maybe you've even brought a couple of monologues with you in case the agent wants to hear you read something by asking you to "show him what you can do." But before it even gets to that, the first part of the interview is usually very awkward. The truth is, the agent at this time has probably already seen your headshot and resumé and has probably even seen you in some production, maybe a local play. But at this time the agent wants to get to know you personally. Hw wants to know how you present yourself to people in a business setting. Since you will be working with the agent in getting you work, he wants to make sure that you handle yourself professionally in a such a setting.
So the agent asks you to tell him something about yourself. What do you say to him? Actually, you should prepare ahead of time for this question. You don't want to be taken off guard. Just as you have prepared everything else, your headshots, resumé, and monologue, you want to prepare for this question. This is the opportunity to get some experience as a writer. Here you will create the exact monologue that you want to describe yourself. You should memorize it and practice it just as you would a movie script until you have complete confidence in reciting it.
To do this, think about what you want the agent to know about you. Don't just spit back your resumé. The agent has already seen that. You can, however, mention a few highlights. If you have good training with a well known school, mention this. Your agent is going to want to know what you've been doing. Tell him about any lead roles you've done. If not a lead, pick a role you've played that you particularly enjoyed playing, even if it was a small part. Talk about what excites you about being an actor. Tell him what you love about theater or television or whatever medium you're most interested in. Tell him about any interesting hobbies that you have. Agents want to see that you're well rounded and not just one dimensional. Well rounded people usually make good actors. Tell the agent what you feel is unique about yourself. Keep your monologue to under a minute. Agents don't have a lot of time and want to get right to the point.
Finally, remember one thing. If an agent has invited you to an interview he is already interested in you so don't push it. Act natural and be yourself. You will find that by not acting you will have your best chance at getting this agent to represent you.