That's the line of code that is calling the function (making it run). The parentheses contain the value that the function is going to work with. In this case, as 10 is divisible by 2 with no remainder, the function will return
If you open up your browser console, and paste that code in, you can test it out by calling the function with different values. Just remember that you won't need to paste the function itself in again, UNLESS, you refresh the page.