It means -- if it's accurate -- that you have enough CYA in your pool without adding more. One way to check is dose with chlorine in the evening, wait, test, and then retest in the AM. If the chlorine doesn't drop much, then you don't have any unusual chlorine demand. Wait again till the following evening, and test again: if it doesn't drop more than 1/3 - 1/2 during a sunny day . . . you have a fair bit of CYA in the pool.