PkgInfo is a standard file that may appear in an OS X/macOS application bundle "file". It is placed in a subdirectory named "Contents". It has long been deprecated, in favor of features of the Info.plist file (see also Property List).
It is just 8 bytes in size, and contains a type code and creator code applicable to the bundle as a whole.
Note that there are a lot of other things named "pkginfo".