Originally Posted by
Brand_new_traveler
On my first flight, the plane left the gate right on time and rolled back and then just sat there for what seemed like forever.
It depends on how long "forever" is.
What you describe is normal if "forever" is a couple of minutes. They don't start the engines until the pushback has begun. As the engines start the electrical source changes from the APU generator to the engine generators and the air source changes from the APU bleed air to the engine bleed air. Taxi can also be delayed by congestion or sequencing based on your departure order.
If "forever" is more than a few minutes then there was likely some problem getting the engines started, generators online, engine bleeds operating properly or some other system working correctly. There are many systems which can't be checked until the engines are running. If something doesn't work initially it must be taken care of before proceeding.