The first number in the parentheses after each gauge example is the number of stitches per 1 inch, and the second number is the number of rows for 1 inch. They just saved you the time of doing the math for you.
For example, in the first one, it says 18 stitches and 24 rows = 4 inches on a size 8 needle. (OR: 4.5 stitches and 6 rows = 1 inch
It is a little confusing the way it was written.