Basically what you're doing is telling the compiler to temporarily override its stong type checking. Normally, if you declare a variable in a function 'int', but then pass an 'unsigned long' to it, you'll get an error (or a warning at the very least) informing you there's a type mismatch. The problem here is that OpenDevice(), being a very general function to deal with any device, only accepts general IORequest structures. But in your program, you're dealing with SerialIORequests, AudioIORequests, SCSIIORequests, and what not. Passing such a structure to OpenDevice() would cause the compiler to flag an error, so you tell it: 'Look, right this moment, this SerialIORequest has type IORequest. Trust me, I know what I'm doing, so quit complaining.' You could even cast it to void *, or char * or somesuperblynewflashytype * if you wanted (and the function was defined with that type, otherwise you'll still get a type mismatch error).
As iamaboringperson said, you'd do well to start reading a book on ANSI-C. Get the (expensive but) definitive guide by Kernighan and Ritchie, 2nd edition. Worth every last penny.
