An XCMD is a way to extend the number of commands that HyperCard understands. By writing a small routine in Pascal and inserting it into HyperCard, you can cause HyperCard to execute it by simply entering that routine's name into your scripts.
More information on XCMDs can be found in the "HyperCard Scripting Manual" and demo disks, availble from APDA. The Appendix is especially useful when learning to write your own XCMDs.
(NOTE: HyperCard also allows you to write XFCNs - external functions - that facilitate returning values to your script)