If images are made up of many tiny pixels, the resolution of an image is determined by their spacing. High resolution, such as 300 dpi, will show more detail in a given photograph. Colors and subtle features will also be more clear in a higher resolution image. The monitor of your computer is likely around 72 dpi, and it may be fine for the type of work you need to do. But, a photo that is only 72 dpi is not likely to suffice for many other purposes. For quality use of stock photos, you'll want a much higher dpi.