Question

If a person scores in the 45th percentile on the ACT, how do we interpret this?

Answer 1

It means that about 45% of the people did worse, and about 55% did better.

You can 'carve up' your population according to score. You probably now the term median , which is the value of the middle of your population (when arranged by value).
Quartiles are also widely used, they divide the (ordered) population in four groups. If you're in the second quartile that means that at least 25%, but possibly 50%, did worse than you.

And then comes the percentile: you divide your ordered group in 100 subgroups. If you're in the 45th percentile, then 44-45% of the population did worse than you, and 55-56% did better.