If your pool was installed in an area with any water table, the extra hole next to your fiberglass pool should be a well point or sump tube. The tube gives you access to put a pump down and pump water from below the swimming pool away from. This way it doesn’t affect your pool stability in the ground. We have also seen these used near vinyl liner or gunite pools in extreme cases where there is a concern the weight of the pool may not keep the pool in place and counter against hydrostatic pressure. Here’s another great article about dealing with ground water.