The strip image is the wide banner that runs across the top of an Apple Wallet pass in the classic banded layout. It is a file named strip.png inside the PKPass bundle, and it is the single largest piece of brand surface a store card, coupon or event ticket has. On store cards and coupons the primary fields are drawn on top of the strip, so the image and the text share the same space.
Apple expects the asset at multiple scales, with strip@2x.png and strip@3x.png alongside the base file, and the correct dimensions depend on the pass style and on whether the pass shows a square barcode, which shortens the strip to make room. Supply the wrong aspect ratio and the wallet crops or scales it rather than rejecting the pass, which is why strip images so often look subtly wrong in production.
Design constraints follow from the overlay. Anything important, a logo lockup, a face, a product, has to survive text sitting over it, so busy artwork and high-contrast detail in the field area work against the pass. Flat color fields, generous negative space and a foreground color chosen against the image are what make a strip read at a glance in a wallet stack.
The Google Wallet equivalent is the hero image, which fills the same role on the other side of the wallet divide. On passes using the poster layout, the strip is replaced entirely by full-bleed artwork.