To reverse (a vehicle) from a confined space.
He backed out of the garage.
To withdraw from something one has agreed to do.
She backed out of organizing the fund-raising.
To dare (someone) to not withdraw from a challenge.
To bet on someone losing.
To undo (a change).
I had to back out the changes made to the computer when it became apparent that they had stopped it working properly.
To exit a mode or function.
I chose that menu option by accident, so I pressed Escape to back out.