if (n % 2 == 0)" would run if "n" was even ("n" can be divided evenly by 2). "if (n % 2 == 1)" would run if "n" was odd. The only two values that can come out of "'x' % 2" are 1 and 0. Which you should use depends on what you trying to do.
For example if you want to print all the odd numbers from 1 - 50 you would do something like this: