What does it mean by DN in salt-pepper noise? What is DN on page 245 of these notes shown in the slide below?
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).