Here's what I see in that.
For a given is the cardinality of the set divided by .
Since you have m values you count how many of them are such that for a given y holds. Obviously that number is somewhere between 0 and m. Take that number and divide it by m and you get the which then obviously is always a number between 0 and 1.
can be thought of as a percentage provided you multiply it by 100.