When you arrive at security, the TSA agent will scan your mobile boarding pass (or ask you to hold your phone over the scanner) just as they would scan a paper version. Ditto at the gate. You will still need to have a form of identification (drivers license, passport) to pass through security.
Are you checked in once you have your boarding pass?
If you have no hold luggage and check in online, check in is complete when you print your boarding pass. If you check in online and have hold luggage, it's complete once the airline has accepted your luggage. In both cases, you must comply with any deadlines the airline imposes.
What happens after you get your boarding pass?
Should I check-in online or at the airport?
For a short-haul flight where you are only taking hand luggage, checking in online saves a lot of time and is therefore much better than checking in at the airport. If, on the other hand, you have to check in bulky luggage, it is better to use the classic check-in at the airport for security reasons.
How long before my flight can I print my boarding pass?
within 24 hours(Example: Airline Confirmation# A1B2C3) You will be able to print a within 24 hours prior to your departure. You can also typically print from airport kiosks at the airport when you arrive on your travel day. Some airlines may charge a fee for this service, but most typically do not.