Having a ticket to an event allows you access to that event. Tickets cannot be obtained for another event at the same time, meaning you can't buy a ticket to two events that are being hosted at the same date/time.
Most events allow you to purchase up to two tickets, and some even allow you to buy more. If you're coming with a friend, you can get them involved in the action too!
At the start of the event, you turn in your ticket to the event organizer. They, in turn, turn the tickets into Gen Con. This is how attendance is tracked to any given event and how the event organizer gets reimbursed and can plan for future year's spaces.