Program is a textual description of the model tree that allows the addition of some statements that affect regeneration status and parameter values/passing. There's no setting (through WF5) to see and edit the program. Everything you can do with Program could be done entirely manually.
http://help.ptc.com/creo_hc/creo30_pma_hc/usascii/pma/fundamentals/About_Pro_PROGRAM.html
It isn't a general purpose programming language; you can't use it to create new features or create loops.
You can use it as part of a top-down design system to allow coordinating various sub assemblies and parts from a top level.
One use I've made of it is to create a parameter, add that parameter to a family table, and use that parameter in Program to toggle features/components on and off, instead of adding dozens or hundreds of separate items to the table. Changing the location of the IF/ ENDIF statements in the Program controls which items are included or excluded according to one parameter in the family table.
Other uses are more clever, but there's too much to put into this response, hence the link above. If there are more specific questions, ask away.
