An S-Expression, or symbolic expression, is a notation used primarily in the programming language Lisp. It represents data and code in a parenthetical format, where elements are enclosed in parentheses. Each S-Expression can be a single atom, like a number or a symbol, or a list of other S-Expressions, allowing for complex structures. S-Expressions are essential for expressing both code and data in a uniform way, making it easier to manipulate and evaluate. This simplicity enables powerful features like macros and facilitates the development of AI applications, as they can represent complex data structures in a straightforward manner.