What john said. Equalization
involves putting an excessive voltage on batteries, which in turn causes the electrolyte to boil/bubble more vigorously. Which also pretty much ensures you will need to add some water
afterwards to top them up. Make sure you are only using DISTILLED water for this, it is only a buck per gallon in the bigbox stores. Two bucks in the "ironing" supplies at supermarkets.
And it is normal for voltage to fall just a little when you top up the electrolyte, until the next charge cycle.
If you peek into the battery before you start, the electrolyte level should also be full (covering the plates fully and then there's normally a mark for correct level) BEFORE you start the equalization
This is also the time when they will produce the most hydrogen gas, so the smoking light should be extinguished and ventilation provided, just in case.