Basically, it's a hole in the ground beside your pool from which you can pump ground water. All sorts of methods to build it. I would dig a hole a little deeper than your pool, insert an 8 or 10" pvc pipe with several holes drilled in it and covered on the outside with landscape fabric. after inserting the pipe, fill the rest of the hole with #57 (driveway) rock.

That's it. Cut the pipe off flush with your lawn and put a landscape lid on it. If it fills with water, a $60 pump from HD/lowes will easily drop down into the pipe and drain the water.

My neighbor had one built into his decking.....looks like one of the skimmer access lids.