There are two holes in the bottom of my skimmer. I know that one hole leads to the pump suction, what is the other one?

The other hole in the bottom of the skimmer is known as the “equalizer” and connects to the main drain at the bottom of the pool. If the water level of the pool should drop below the skimmer opening in the side of the pool, water will flow into the skimmer from the main drain to prevent the pump from drawing air and losing its prime.