We next arrive at make-dispatch-macro-character. I would suggest that the new Lisp user review the discussion on readtables and read-macros that can be found in the gensym and get-dispatch-macro-character articles.
The make-dispatch-macro-character function creates a new dispatching macro character (i.e. the first character of two-character read macros) for the read table. It is initially constructed with all sub-characters set to signal errors. Once make-dispatch-macro-character has built this new dispatch, the individual sub-characters can be assigned with set-dispatch-macro-character.