Back in the original days of DOS, games ran on specific resolutions on CRT monitors (and occasionally televisions). Limited hardware required these games to run on what is today considered extremely low resolutions. A Scaler is like a filter, that reprocesses the emulated screen before it is drawn on the physical monitor, and enables old games to take on a slightly updated look (or stylized look).
These screen shots (of Doom) represent a small section of the full game screen, and have been blowup 200% to better show how the image is being altered. Some effects are more subtle or visible depending on the range of colors displayed on the screen at individual moments.