Cyanuric Acid (CYA) is stabilizer.

It slows down the breakdown of chlorine, especially by UV rays. Without it your chlorine can vanish in as little as 15 minutes.

But it's a two-edged sword. It slows down chlorine breakdown in EVERY way, including what you want chlorine to do---sanitize your pool.

There's a chart, called the "Best Guess" table that tells you how much chlorine you need depending on your CYA level. The more CYA, the more chlorine.