When you first arrive at Port Canaveral, your party is given a card with a number on it. This is your boarding number. After you've checked in, you can sit down, walk around the terminal, etc until your boarding number is called. Then, you can board at your leisure.
It used to be that after you checked in, if you wanted to get onboard early, you had to go stand in a LONG line for an hour or more, waiting until they started the boarding process.