It'll is the contracted form of 'it will' used as a pronoun-subject followed by the contracted auxiliary verb will. In speech it typically functions as a future marker in informal contexts. Phonetically, it encompasses a strong /ɪ/ or reduced vowel in 'it' with a light, unstressed /l/ and a reduced /w/ in the contraction, often realized as /ɪtəl/ or /ɪtl̩/ depending on liaison to the following word.
Full pronunciation guide
