Resolution
In this tab you can set the screen resolution in which your game must run. By default the resolution is not changed. But sometimes you want to run the game in a lower resolutions or you want to set the frequency of the monitor to make sure the timing in the game works correctly. If you want to change the resolution you must first check the box labeled Set the resolution of the screen.There are three things you can change. First of all there is the color depth. This indicates the number of bits used to represent the color for a pixel. Most machines now only allow for 16-bit (High Color) or 32-bit (Full Color) but older machines also allowed for 8-bit and sometimes 24-bit color. Game Maker only works correctly in 16-bit and 32-bit color. 32-bit color gives nicer looking images but will take more memory and processing time. If you want your game to run will on most older machines set the color depth to 16-bit. Otherwise use 32-bit or don't change it.
Secondly there is the screen resolution, the number of pixels (horizontal and vertical) on the screen. Changing the resolution is useful when e.g. your rooms are very small. In this case it might help to reduce the resolution of the screen. Realize though that this will also effect other applications running. This can in particular give problems with low resolutions. So in general it is better to only do this when running the game in full screen mode. Game Maker will automatically change the resolution back to the starting situation once the game finishes.
Finally you can change the refresh frequency. This indicates how many times per second the image on the screen is refreshed. If your room speed is larger than the frequency not all steps are actually visible. It works best if the frequency is a multiple of the room speed. (If you specify a frequency that is too high or not available the frequency is not changed.)
There also is a setting here to Use synchronization to avoid tearing. This requires some explanation. A display is redrawn a number of times per second, depending on the refresh frequency. If a room is drawn halfway such a refresh, the top of the display will still show the old image while the bottom part shows the new image. This is called tearing. To avoid this you can check this option. In this case the new room image is only copied to the screen when the refresh is not inside the window avoiding the tearing most of the time. The disadvantage is that we normally have to wait till the next refresh. This means that the maximal number of frames is bounded by the frequency of the monitor and, when the processing is not fast enough, the framerate immediately drops to half that number. Also there can be a conflict between the internal timing of the game and the synchronization. If you want to do this best set the monitor frequency to e.g. 60 and also make the room speed either 30 or 60.