The resolution of an image refers to the amount of detail in the image; the higher the resolution, the more detailed the image.  To make sure your printed piece turns out great, black and white line art should be scanned as line art or bitmap at 600 dpi.  Photos, color or grayscale, should be scanned at 300 dpi for standard spot or process printing.  This is true for digital or offset printing.  If your image is less than 300 or 600 dpi, you should replace it with a new image instead of trying to fix it.  To guarantee the best quality, do not enlarge images beyond the original size in the design process.