Pixels Per Meter is a measurement used to define the amount of potential image detail that a camera offers at a given distance.
Digital images are divided into grids of pixels or blocks, each one containing the color for that specific point in the image. The PPM value defines the number of points or divisions for each meter of the subject that the camera is imaging. A higher PPM value means that the image has more detail capacity. Note that a PPM value must always be tied to a distance. For example, our 2075mm lens on a 2MP 1/2.8" sensor will give 286ppm of detail at 2.5km, 89ppm of detail at 8km, or 16.2ppm at 44km.
It is important to know that while PPM defines the number of pixels of detail that a camera can deliver at a given distance, it does not necessarily define the amount of sharpness or contrast in that image, as those aspects will also depend on the quality of the lens, the distance to the subject and the atmospheric conditions between the subject and the camera. Two cameras with the same PPM may offer different levels of detail depending on how sharp the lens is, and likewise two cameras looking at two different distances with the same PPM will likely have different amounts of perceived detail since the longer-range camera has more atmosphere to look through.
That being said, knowing the PPM of a camera at a given distance will give you a very close initial estimate of detail, and thus in general, PPM is a good way to quickly assess the potential performance of a camera system.