You've got chloramines.

It means your pool’s chlorine has already done its job on contaminants but hasn’t been refreshed. Free chlorine is the active sanitizer—zero means nothing is left to kill algae or bacteria. Combined chlorine (chloramines) is the waste product that smells like bleach and irritates eyes and skin.

You need to shock the pool. Adding more chlorine will break down the chloramines—just enough to get the free chlorine level up to about 10x the combined chlorine level. Test again after a few hours. If free chlorine holds, you’re good. If not, something in the water is eating it faster than you can add it.

That “pool smell” people complain about? That’s not chlorine—it’s chloramines. A well-chlorinated pool has almost no smell. So fix this before anyone goes swimming.