That'll is a contracted form of 'that will' used to express future action or obligation in informal speech. It functions as a modal-like auxiliary with reduced pronunciation, typically realized as /ðæˈtɔl/ or /ðæˈtəl/ depending on dialect, and often blends the /l/ into a syllabic or weakly articulated ending. It marks future intent in conversational English and is common in natural, rapid speech.
Full pronunciation guide
