Help!! (Another Italy Train question)

Hey everyone

I am traveling to Rome tomorrow. When I land in FCO, I have been instructed (by the person I am going to meet) to purchase a train ticket. Then he mentions something about "validating the ticket". What does this mean? Also, if anyone can "walk me through" the process of how to get from the terminal to wherever the train departs, where to buy the ticket, I would be SOOO thankful! I cannot get ahold of my friend.

Thanks!