The "strip" (also called "tagging") is a clear coating that is used by the USPS mail sorting equipment to sense where the front and the upper right corner of the mail piece is so that it can be faced properly for postmarking and reading of the address. Under longwave UV light it will glow kind of an greenish-yellow. Stamps are tagged that way as well, but usually with an "all-over" coating, so you don't see it as such