DPI stands for Dot Per Inch. 300 dpi is 300 dots per inch and 600 dpi is 600 dots per inch.
DPI is about resolution. The high the resolution (DPIs) the better the quality. 600 dpi has a better result in comparison to 300 dpi.
600 dpi scans will provide as much detail as possible but will result in a larger file.
600 dpi is best for creating new prints, enlargements and digital photo books. 600 dpi will allow you to double the size of a photo (for printing) without loosing detail. No additional detail is added.
Slides and Negatives
Slides and Negatives are scanned at 4800 dpi.