They'll is a contraction, a shortened form of two words squeezed together: “they” and “will.” The apostrophe shows where letters have been left out. When you say they'll arrive soon, you're saying “they will arrive soon” in a quicker, more natural way.

People use contractions like this all the time in everyday speech and informal writing. You might tell your friend, “They'll love your science project” instead of the more formal “They will love your science project.” Both mean exactly the same thing, but the contraction sounds more like how people actually talk.

You'll see they'll used to talk about what people (or things) will do in the future: “They'll start the movie at seven o'clock” or “They'll need to bring their own supplies.” It works for any plural subject or group: your classmates, the neighbors, a pack of wolves, a collection of books. Anything that can be called “they” can become “they'll” when you're talking about what will happen next.