Retrieving What variables mean when retrieving view definitions?
a and c are table aliases; they're defined in the FROM clause of the SELECT statement:

...
FROM (((actor a
        LEFT JOIN film_actor fa ON ((a.actor_id = fa.actor_id)))
       LEFT JOIN film_category fc ON ((fa.film_id = fc.film_id)))
      LEFT JOIN category c ON ((fc.category_id = c.category_id)))
...


a refers to actor, c to category, etc.

It's a little hard to follow until you notice that there's a sub-query involved inside the outermost group_concat function.