what is the AC power ?

Not sure what you mean by the AC power when you update an MBP...the MBPs use a charger to convert the AC power from 100-240 volts at 50-60 Hz to the DC power the MBP needs to run/charge the battery. The power rating of the charger will be over a range depending on the particular MBP...they use various power ratings for the different models of MBPs.

It has to be plugged into the wall during updates.

If an update was done on battery power only and the battery became discharged during the update you could end up with a non functional machine.

AC power is what most people in the US call "the mains" according to many other parts of the world. This is your external power adapter that charges the battery and keeps your system running if the battery is depleated.