Answer:
90th percentile is simple another way of saying that 9% scored above and 90% scored below (the other 1% being the number 90).
For example, if you take a type of standardized test in high school, if you do well, you are at the 90th percentile, meaning 90% of students scored lower than you and 9% scored better. You can not be in the 100% percentile, because you cannot score better than yourself.