It is well known that Android uses the Linux user system as simple sandbox system to separate the apps form each other.
As far as I remember the part after the underscore is simply the Linux user id in hex representation.
The user accounts are created dynamically on app installation and then saved to a XML file where this and many more details if all installed apps are recorded. This way Android can lookup which app belongs to which uid and the other way round.
For more details on this topic see the related questions and their answers: