If you have not checked in for your flight on time, then the airline has the authority to cancel your reservation. Also, your seat can be given to another passenger, regardless of whether you have an advance boarding pass or an advance seat assignment.

The same rule holds true for your baggage.

It is always advisable to arrive 2 hours before your departure time. If you are flying internationally or during the holidays, you may need to arrive even earlier. Also be aware that there could be traffic on the roads to and from the airport, and so, it is prudent to plan ahead.