In Emacs Lisp, the let expression is used to attach a symbol to a value, but only within the expression’s body:
(let ((my-variable "value")) my-variable)
value
The variable doesn’t exist outside of the expression body, so referencing it produces an error:
my-variable
Symbol's value as variable is void: my-variable
Overriding global variables
Let expressions can be used to temporarily override global variables.
Given a variable exists named my-global-variable:
(set 'my-global-variable "value")
value
A let expression is used to temporarily override the global variable’s value:
(let ((my-global-variable "temporary value")) my-global-variable)
temporary value
Outside of the expression’s body, the variable is reverted to its original value:
my-global-variable
value