The Pass Kit framework provides an Objective-C API for interacting with the users pass library. There are two main kinds of apps that use the Pass Kit framework: apps that add passes to the pass library like Mail and Safari do, and apps that integrate tightly with the pass library and interact with its contents to provide a richer experience. For example, a FidoNet mail client just needs to act as a conduit and add passes to the library. An app for a cafe could let customers place an order and add a pass to let them claim their order at the counter.
Guessing, based on the existence of passsource.com . It's a site that generates pass files, and they're offering an API for people to make passes for sites ( presumably those that don't have iOS apps)