One layer is being used as a track matte for another layer. Read up on Track Mattes here.
Track Mattes are one of the very basic things you should know about working with AE. If you don't know what they are, there must be a TON of other useful things you've missed. I would highly recommend that you go through these resources before you do much more in AE. It will help you work with a lot less frustration.
Thank you very much!
No problem. Glad to help!