When chlorine is added to the pool, it takes the form of Hypochlorous Acid (HOCl).   Unfortunately, Hypochlorous Acid is very
unstable in the presence of sunlight and dissipates very quickly.  The result is that chlorine will quickly disappear from an outdoor pool on a sunny day . . .
unless you have a proper amount of stabilizer in the water.

Stabilizer is a chemical that is added to the pool water to help to keep the chlorine from being used up too quickly.  

Stabilizer works by slowing down the chlorine reaction in the water.  The conversion rate of the chlorine is reduced
by a factor of 5 to 10 times (Merck).  This can be both good and bad.  At the proper levels (30-60 ppm) it will be effective at keeping the chlorine
consumption to a reasonable level.

There are a couple of problems that can result from having improper Stabilizer (Cyanuric Acid) levels.

If the stabilizer is too low, then the chlorine will be rapidly lost from the water in the presence of sunlight. 
While the chlorine is in the water, it will be very effective, but the chlorine usage of the pool will be inordinately high.

If the stabilizer is too high, the chlorine usage in the water will remain at reasonable levels, but the
effectiveness of the chlorine will be greatly reduced.  This is because the stabilizer forms a stronger bond with the water and interferes with its ability to
do its job.

The topic of high stabilizer is a matter of great debate within the industry.  Click
here to read more about this debate. 

Normally, the stabilizer level should be maintained between 50-80 ppm.  If the stabilizer level exceeds 100 ppm, then
some action will need to be taken to insure that the chlorine remains effective.  This can be done by draining and refilling the pool, or by using chemicals to
counteract the over-stabilization.  See the Stabilizer-Adjusting section for more information.