A copybook is a PDS member that contains valid COBOL stmts. These stmts will execute (or be interpreted by the Compiler - they can be data definition stmts, pgm code, FDs, etc. ) as if they were written individually in the code. You just have to code "COPY copybkname." on a line in the pgm and all the stmts in the copybook become part of the pgm.
The JCL copylib is the PDS where the copybook members reside.