They have the same meaning. 'Till' is less formal than 'until' so it's not used in writing. 'Until' is used in writing and can also be used in speaking. 'Un' here is not a prefix and has no unique meaning.

Hey! They have the same meaning. “Till” is not used as much as “until” in contemporary English. Till is the “Old English” version of until. But they can be used interchangeably.