Resolution refers to the number of pixels in an image. Resolution is sometimes identified by the width and height of the image as well as the total number of pixels in the image. For example, an image that is 2048 pixels wide and 1536 pixels high (2048X1536) contains (multiply) 3,145,728 pixels (or 3.1 Megapixels). You could call it a 2048X1536 or a 3.1 Megapixel image. As the megapixels in the pickup device in your camera increase so does the possible maximum size image you can produce. This means that a 5 megapixel camera is capable of capturing a larger image than a 3 megapixel camera.
How does image resolution play out on my computer monitor?
The computer screen you are looking at right now is set at a particular resolution as well. The larger the screen, the larger you likely have your screen resolution set. If you have a 17" monitor, likely you have it set at 800X600 pixels. If you have a 19" screen it is likely set at 1024X768. You can change the settings but these are optimum for those screen sizes.
Now, if your monitor is set to 800X600 and you open up an image that is 640X480, it will only fill up a part of your screen. If you open up an image that is 2048X1536 (3.1 megapixels) then you will find yourself moving the slider bar around to see all the different parts of the image. It just won't fit.