Fold
Crease
noun
A fold you can see with your eye is a fold or crease in something.
The readings are both kun'yomi vocab readings. You should have seen 折る by now which should help you to read 折り. Then there's 目, which I'm guessing you won't have a problem with.
彼はゼッタイに折り目をつけずに本を読みます。
He reads books without ever creasing the pages.
折り紙を半分に折って、折り目をつけてからまた開いてください。
Please open the origami again after you fold it in half to add the crease.
ズボンには折り目を付けてもらえますか？
Could you put a crease in my trousers?