How Does the Concept of a “Trapdoor Function” Relate to Cryptographic Security?

A trapdoor function is a function that is easy to compute in one direction but extremely difficult to reverse without a piece of secret information, the "trapdoor." While SHA-256 is a one-way function, it is not a trapdoor function because there is no secret key to reverse it. Trapdoor functions are mainly used in public-key cryptography, like RSA, not in hashing.