Parts per million. It means one part in 1 million parts; for example, one pound of chlorine in 1 million pounds of water. The acceptable ranges of most water tests are measured in ppm. For instance, free available chlorine should always be kept in the 1 to 3 ppm range while total alkalinity needs to be kept in the 80 to 120 ppm range.
The pool owner found the ppm measurements to be an easy way to understand the results of his tests.