The hero image is the wide banner on a Google Wallet pass. It is set as a field on the class, where it applies to every pass of that type, or on the individual pass object, where it can differ per member. Google hosts the image from a URL you provide, so it is referenced rather than packaged the way Apple Wallet bundles its assets.
Google renders the hero image at a wide aspect ratio, roughly three to one, across the card. Unlike Apple's strip image, pass fields are not laid over it, which frees the artwork from having to leave room for text but also means it carries less of the pass's identity on its own: the logo, background color and field text sit around it rather than on it.
Because the hero image is a hosted URL, it can be changed after passes are issued. Updating the class image reaches every pass of that type, and updating an object image reaches one member's pass, which makes it a genuine campaign surface: a seasonal banner, a store opening, a tier upgrade. Passes already in wallets pick up the new image without the member doing anything.