Simplified Common Lisp reference
Symbol class: Functions, Evaluation, Flow Control, Definitions and Syntax
Symbol type: special form
letbindingsbody(zero or more) => an object
Argument description:
bindings list of variable - initial value pairs
body program code in which definitions above are effective, implicit progn

LET is special form for variable binding. Bindings are described in two element lists where the first element specifies name and the second is code to compute its value, or single variable without default initialization. There are also declarations possible before body.

(let (a b (c 3) (d (+ 1 2))) (list a b c d)) => (NIL NIL 3 3)
Function indexFull documentation for let (HyperSpec)