“ppm” stands for “parts per million.” It is a unit of measurement used to quantify the concentration of chlorine in water. Here's a bit more detail:

1 ppm indicates that there is 1 part of chlorine per 1 million parts of water.

In practical terms, this means that 1 ppm is equivalent to 1 milligram of chlorine per liter of water (mg/L).

For example, if you test water and find that the chlorine level is 2 ppm, it means there are 2 milligrams of chlorine in every litre of water.