The third element of a list. (caddr list) is short for (car (cdr (cdr list))); car brings you the first element of a list, while cdr brings you the remainder (after the first element) of a list.
Lisp defines all combinations matching the regular expression C[AD]+R up to at least 6 letters. Thus CADR is the second element of a list, CAAR is the first element of the first element of a list, and CDADDR is all but the first element of the list that is the third element of a list.
If you decide to node all these words, the EDB and I shall personally hunt down and eat you.