The short form of "you will", and occasionally of "you shall" (that second one is old-fashioned and you can ignore it). It is completely normal in speech and in friendly writing — emails to friends, messages, novels. In very formal writing, such as an academic essay or an official report, people usually write "you will" in full. Two things worth knowing. First, the apostrophe stands for the missing letters w-i, and it is not optional: "youll" is simply wrong. Second, in fast speech it often sounds like just "yull" or even blends into the next word, which is why learners hear a sentence like "You'll be fine" and think a word is missing. Remember that "you" here can be one person or many people, and it can also be the general "you" meaning "anyone": "You'll find the hotel on the left" (you specifically) versus "You'll never know unless you ask" (people in general).