This is an indication of colour usage. The numbers represent the number of colours printed per side of the paper. The first digit is the front, the second digit (after the /) is for the back. So 1/1 is printed 1 colour on both sides. Full colour = 4 colours.
4/0 = full colour only on the front side
4/4 = full colour on both sides
For more explanation of terms, check the glossary.