This is a contraction of "it will". People use it in casual spoken English. Don't use it in formal writing unless you're trying to quote someone who was speaking casually.

His landlord reassured him that it was only a temporary inconvenience. "It'll be ready by Monday," he said.

"It'll" is similar to "this'll" and "that'll". All of these contractions are less common and less formal than the usual contractions like "can't" and "isn't".