The resolution of an image refers to the density of the pixels (or printed dots) that make up that image or graphic. The higher the resolution, the crisper and more detailed the image will be. A lower resolution image will be fuzzy and less detailed.
Resolution is measured in DPI (dots per inch). In actual fact, DPI only refers to the number of printed dots in a square inch of a printed image. When viewing an image on screen, the resolution is referred to as PPI (pixels per inch) as pixels are used to make up an on screen image and not dots. However, generally people never use the term PPI and refer to both on screen and printed resolution as DPI.