What does ppm mean?

ppm is an indication of concentration. Directly translated, it means "parts per million" i.e. number of parts per million. The unit is often used at very low concentrations.

For example, an indication of 10 ppm means a concentration of 10 parts of the substance of 1,000,000, or converted to % = 0.00001% - so it's easier to write 10 ppm.