When a package is defined the definition of it is saved in the database.
For every file that belongs to a package a record in added to the table PACKAGE_FILES_HASH.
The hash field of every single file is empty when the package is created.
The first time that the package runs the hash will be calculated by the scheduler.
RESET PACKAGE HASH
What the "Reset Package hash" does is simply remove for all the files belonging to the package the HASH field.
The scheduler will recalculate it when the job will run.