it stands for Dots Per Inch
meaning the number of printed dots per inch of the photo.
u'll only need this mainly for printing purposes.
ultimately, it is the image dimensions that counts.
for eg.: ur image is 3000 x 2000 (3000 pixels by 2000 pixels)
the DPI shows lets say 240.
it means the physical size is 12.5" x 8.33" if it is printed at 240DPI. (divide the pixels by 240)
so if u change the DPI to 300.
it means the physical size is 10" x 6.67" if it is printed at 300DPI. (divide the pixels by 300)
does it make a difference? the image dimensions are still 3000 x 2000.
it only lets u know wads the physical size.
u can still print it at different sizes specified by the printer.