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.