MLKEM768-X25519 is a hybrid post-quantum encryption algorithm meant to combine the strengths of the classically-strong X25519 and the (supposedly) quantum-safe MLKEM768.
KEMs are key encapsulation mechanisms, which allow two people to come up with a shared secret in unsafe environment.
the reason why people tend not to use MLKEM768 by itself is because it’s new and therefore not as battle tested as things like X25519. therefore, if something were to go wrong with it, it’s at least as safe as X25519 is.