It'll is the short form of it will. You use it to talk about the future, or to make a promise or a guess: It'll rain later. It'll be fine. It is very common in speech and in casual writing. In formal writing, people often put it will instead. The 'll is the same piece you hear in I'll, you'll, we'll. It'll is not it's (it is / it has), and it is not its (belonging to it). Those three sit next to each other in students' books and cause a lot of quiet trouble.