Pools use chlorine to kill algae, bacteria, and other organisms that live in the water to keep them clean. The chlorine acts to displace oxygen from the water molecules, and it's actually oxygen that kills the unwanted organisms that try to grow there. It's important to keep the chlorine levels high enough to keep the pool sanitary.
Use test strips to test the water at least two to three times a week, and test daily, if possible. The test strips should show your chlorine levels as being somewhere between three and five parts per million (ppm) and your cyanuric acid levels are between 25 and 50 ppm. Read the test strip package's directions to determine how to interpret the results.
Ensure that your chlorine dispenser has chlorine in it, if you add chlorine directly to the pool, either in tablet or granular form, depending on what it uses.
Ensure that your saltwater chlorine generator, if that's how you chlorinate your pool, is set to a high enough voltage to generate sufficient chlorine daily and that you run it once a week on the super-chlorinate cycle to shock the pool and bring the levels back up. Not super-chlorinating every week can cause your levels to fall off over time.
Add cyanuric acid, a chlorine stabilizer, as necessary if your concentration is below 25 ppm, the chlorine level is below three ppm, and the chlorine dispenser or saltwater generator is working properly. Cyanuric acid stabilizes the chlorine level in your pool by preventing sunlight from leeching the free chlorine from it, so that without stabilizer, you won't be able to maintain your chlorine levels high enough. Some chlorine tablets, called "stabilized" chlorine, already have stabilizer in them. If you have these and your stabilizer won't stay above 25 ppm, the dispenser may not be working properly. If you use a saltwater chlorine generator, maintaining stabilizer is even more important than it is with a direct chlorine dispensing system, because salt-generated chlorine tends to leech out faster than directly-added chlorine.