Check the chemicals that are in the pool and the pH level. You should do this about twice a week in the summer. Even though you probably don't swim in the winter, you still need to check the chemical levels so that you're not performing as many tasks when the pool is opened again in the spring. While checking the pH, you might want to add chlorine depending on the levels that you read.
The skimmer in the pool helps to trap leaves, small animals, and other debris that get in the water. Check the skimmer once a week at least to make sure it's clean so that it functions properly. The device is often located on the side of the pool. Some are removable and can be used to go over the surface of the water to remove any debris that is present. The skimmer can be used to remove hair and lint from the water as well.
Monitor the water level in the pool. A little evaporation is expected, especially if the temperature is higher than normal. If you see a significant decrease in the water level, then you want to examine the pool for any leaks. The water level should be at the midpoint of the skimmer as this is a point where it's not too high or too low. Clean the filter regularly. If the filter gets clogged or too full, then you're going to notice the water turn cloudy and have more debris in it than normal. The filter will prevent excessive amounts of water from being wasted and is ideal if the pool is set up near a tree where leaves tend to fall more than if the pool is in an open area.