It’s a convenience macro to save you typing nonnull in your headers. From the Swift blog detailing how new safety features have been incorporated back in Objective-C:
To ease adoption of the new annotations, you can mark certain regions
of your Objective-C header files as audited for nullability. Within
these regions, any simple pointer type will be assumed to be nonnull.
See Nullability and Objective-C – https://developer.apple.com/swift/blog/?id=25