ppm is "parts per million".

15ppm says for each 15 units of CL, there is 1 million units of something else (water).

You need to dilute the pool water to measure over 5. Take one shot glass of pool water and put it in a cup. Now add 4 shot glasses of distilled water (tap water is probably close enough, test to make sure it has no CL).

Now do your test... and multiple by 5. So 1ppm on your gauge is now 5ppm in the pool.

Robert