In very simple terms, the ADIRS tells the plane where it actually is.
The top left three knobs on the overhead panel turn it on.
After you switch them on, it takes about 7 minutes for the ADIRS to self -test and align before it is ready for use.
Don’t move the aircraft till that process has finished.
There is a countdown in the alarm conditions on the upper ECAM that tells how much time is left to go.