What is resolution in computer graphics?

The resolution of an image in computer graphics refers to the pixel density of said image. For instance, older monitors used 800x600 often times. This means that for every 4 pixels in the horizontal direction, there are 3 in the vertical. This is where the aspect ratio comes from, which for an 800x600 resolution, is 4:3. The higher the value of the resolution, the more pixels there are per area of the image or screen, which means that there is more potential for fine detail, and on most systems means a crisper image. If you want to read more, check out the Wikipedia entry about display resolutions:

http://en.wikipedia.org/wiki/Display_resolution