The unit ppm stands for parts per million, and it refers to how much of one substance by weight is present in a mixture with one million times as much of another substance. Only very dilute substances are measured in ppm, such as minor impurities in water or air.
One ppm is the same as 0.0001 percent by weight. With dilute substances, the weight of the trace portion is negligible, so it is equivalent to a concentration of one milligram per liter (mg/L) in a solution with water. Other measurement units for even more dilute substances are parts per billion (ppb) and even parts per trillion (which is not abbreviated, because ppt could also mean "parts per thousand," which would cause confusion).