Rayo’s Number is the smallest number larger than any number that can be named by a formula of fewer than 10¹⁰⁰ symbols in a formal language like set theory.
You can define TREE(3), Graham’s Number, Busy Beaver numbers, etc., all with finite symbol strings. Therefore, they all fit inside the set of “numbers definable with fewer than 10¹⁰⁰ symbols.” Rayo’s Number is one larger than the largest of those. And since its definition itself uses fewer than 10¹⁰⁰ symbols, it’s self-contained and valid under its own rule.