A symbol in Lisp can have a value, also called its binding. There must be a way to distinguish the symbol itself from its value. In English these are not formally distinguished.
The President lives on Pennsylvania Ave.The spouse of the President is named Hillary.
In Scheme, we denote the symbol itself by quoting it with a single-quote symbol.
(address 'president) (spouse president)
Internally, ' becomes the pseudo-function quote:
(address (quote president))
Quoting a list makes a constant data structure:
(define formula '(= area (* pi (expt r 2))))