GETHASH function reads associated value for given key in hashtable. (SETF GETHASH) adds or replaces associated values. See also MAKE-HASH-TABLE.

(defparameter *tab* (make-hash-table)) => *TAB*
(gethash 'x *tab*) => NIL, NIL
(setf (gethash 'x *tab*) "x") => "x"
(setf (gethash 'y *tab*) "yy") => "yy"
(gethash 'x *tab*) => "x", T
(gethash 'y *tab*) => "yy", T
(gethash 'z *tab* 'moo) => MOO, NIL