PPI is pixels-per-inch or points-per-inch. It is used to express the resolution for a screen, say a monitor, a television, a tablet or a smartphone, just to mention a few.
If you think of a game of chess measuring 1 inch by 1 inch, the PPI is 8. You have 8 pixels across and 8 pixels down, giving a total of 64 pixels. So the PPI does not count the total number of pixels per inch, but tells you how many pixels you will find on both sides of the 1 inch by 1 inch square.
The PPI cannot change. The resolution of your monitor or your television has the PPI set from the factory and it remains fixed – no parameter of configuration can change that.
The PPI is a grid into which the pixels in your image can fit. If your image file has a lot more resolution than the PPI, the screen simply cannot do justice to the quality of the file and will have to scale down the resolution of the file to match the PPI of the screen. You can counter this when you edit by zooming in on the image in which case the constant PPI would be applied to a small(er) part of the image and hence would yield a relatively higher resolution.
Typically the PPI increases the smaller the device is. My iPhone 13 has a PPI of 440 approx, whereas the 32 inch AOC monitor I use to write this has a PPI of 140. You would think that the iPhone is a much better screen than the AOC, but here you have to factor in the viewing distance: The greater the viewing distance, the less PPI you will need. I find this to be a bit counterintuitive, but have accepted that it is the way it is. If a billboard was made using a monitor, you would not need 140 PPI’s!
Many want their camera to have a lot of megapixels, and primarily use the files produced to present on a UHD monitor (typically 32″ in size). The resolution is 3840 x 2160 equal to 8.3 megapixels which is much less than say the 24mp you would find in a Nikon Z6ii or a Nikon D750.
Related reading
What is the difference between resolution and bit-depth in photography?
Is JPG a bad format for photography?
