Your monitor is made up of a grid of light emitting squares. If your monitor is 1920x1080, then there's 1920 squares in the horizontal direction and 1080 in the vertical direction. If the game also runs at 1920x1080, then the pixels rendered by the game will line up perfectly with the monitor's physical squares. The game is said to run in native mode then. This is always better because if your game rendered a higher or lower resolution, then the pixels rendered by the game will still have to be fitted into the 1920x1080 squares on your monitor. So the image will either be scaled up or compressed to fit into your monitor. This leads to ugly, unsharp images because the scaling inevitably means having to use an uneven amount of squares. See this example image of this ugly effect
https://en.wikipedia.org/wiki/File:N...n_1024x768.JPG