Simplified Common Lisp reference
Symbol class: Sequences (Lists, Strings) and Arrays
Symbol type: function
stringobject => string
Argument description:
object an object

STRING function converts symbols, characters and possibly some other types into a string. If object is of string type, it is directly returned.

(string 'moo) => "MOO"
(string #\a) => "a"
(string "some string") => "some string"
Function indexFull documentation for string (HyperSpec)