It is just another text file used in program development by many assemblers. It is helpful in systems with numerous modules to be able to include a common set of definitions so that bugs are not introduced by conflicting definitions. There can be a hierarchy of include files so that for example if there are multiple target processors, each module can include a file named "hardware.inc" which then figures out the appropriate definitions for the actual target processor, so that in the module you refer to things with a symbolic name which gets resolved by the assembler into an actual address.
Actually I was just coming over here to remove this thread. I figured it out poking around MPLAB IDE. Figured out what they're for. Thanks for the response though. There was some useful information in it that I wouldn't have known otherwise.