What does it mean by DN in salt-pepper noise?
From the same course, you can find the following indication in an earlier set of slides:


  Input quantized image pixel values (integers): Digital Number (DN)


From this you can infer that


  Salt noise: DN = maximum possible


indicates that salt noise shows up as pixels with the largest possible pixel values (e.g. 255 if the picture is a grey scale picture represented by pixel in the [0-255] range), and that


  Pepper noise: DN = minimum possible


indicates that pepper noise shows up as pixels with the smallest possible pixel values (e.g. 0 if the picture is a grey scale picture represented by pixel in the [0-255] range).