Closing a pool means preparing a swimming pool and the related equipment for wintertime and freezing temperatures. This involves a deep cleaning, lowering the water level slightly, adding chemicals, and draining plumbing.
If you live in an area that's warm all year, you may only need to cover your pool when you're not using it. If you live in a colder climate, you should close your pool when temperatures drop below 65 degrees and before they hit freezing levels.