What does ppm mean?

ppm is a concentration indication. Directly translated it means “parts per million” i.e. number of parts per million. The device is often used at very low concentrations.

An indication of e.g. 10 ppm therefore means a concentration of 10 parts of the substance in 1,000,000, or converted to % = 0.00001% – then it is easier to write 10 ppm.