Let's dive into a quick but clear example.
Take a percentile measure of 60, say. In a particular group, this shows that 59% of the students have performed more poorly than that student. So, a student in the 99th percentile is the top performer in the class for that particular test/period/etc.
This is useful to know, especially when your child has an average of say 80% but is in the 50th percentile.
This shows that the child has done well with the test, but they are in the middle of the group. That means that an 80% average is relatively low compared to the rest of the group. This means that in the same group if a child sits in the 85th percentile, their average is very high and is much higher than 80%.