CYA is cyanuric acid, also called isocyanuric acid or sometimes stabilizer or conditioner. It is kind of like sunscreen for your chlorine. If you don't have enough, then your chlorine is quickly lost to the sun. Your needed chlorine level for your pool is dependent on what your CYA level is so it is an important reading for us to know to give you good advice. The CYA test is not one that you need to run very often unless you are either adding CYA directly or are using a stabilized form of chlorine such as trichlor pucks or dichlor shock powder. If you haven't already done so, take a look at the Best Guess Chlorine Chart in my signature below. Hope this helps.