parts per million it figures out how much dissolved solids is in the water. like reverse osmosis water is really low like around 5 or so ppm. tap water can range up way higher and it tells you how much metal and stuff is in the water and other things. usually tap water with higher ppm will have calcium and mag and other metals in the water. when you use fertilizers they will raise the ppm so you can get an idea of how much ferts are in the water or how much stuff is built up in your run off water. alot of ferts have a ppm chart along with the feeding schedule to give you an idea of what the ppms should be after adding their nutrients. i think with tap up to 150 ppm is pretty good the higher you get the worse it could be for your plant. then you can check how much other things your adding in like i said.
also there are EC meters which do the same thing but on a different scale i believe. i do not have an EC meter yet but i probably will after i find the funds later on down the line. probably invest in a good meter that does everything for 100-200 bucks. just make sure you keep your meters calibrated especially cheap ones if they come with the ability to be calibrated.