A Ticket Is Just a Piece of Paper
Just because you have a ticket and a reservation, don't think you'll head out on your intended flight. Overbooking is a common practice among airlines. It's an attempt to cut losses (or make profits) from passengers who don't show up. So your ticket is no guarantee that you'll get a seat.
If your flight is overbooked, the U.S. Department of Transportation requires the airline to ask for volunteers to be "bumped" from the flight. It'll get you on another flight, and the airline will offer some form of compensation -- perhaps expenses for meals while you wait, cash or a ticket voucher for another trip. If you're bumped involuntarily, the airline has specific DOT rules it must follow, including giving you cash compensation if its overbooking means you'll arrive more than an hour late at your destination.