They relate to the number of screen pixels drawn across and down your screen e.g 1024 across x 768 down. On a 15" TFT screen this is a good resolution and is usually what the monitor is designed around. On a 15" CRT monitor which has an actual visible diagonal of around 13.25" it will result in icons and text that are just a little too small for anyone other than the eagle-eyed youth.
The larger the resolution, the more that can be fitted on the screen but for a given size of screen, the icons and text become progressively smaller. However on 17", 19" or 21" screens then these resolutions come into their own as there is a larger area to be drawn on. Indeed these larger TFT monitors are designed to give their best at these resolutions.
Hope this helps a little.