You is the word we use to refer to the person we're talking to. When your teacher says “You did a great job on that project,” she means you specifically, the person listening. When a sign says “Please wash your hands,” it's addressing you, the reader.

Now we use you whether we're talking to one person or a whole classroom, and whether we're being casual or respectful.

You can also be used more generally to mean people in general, not anyone specific. When someone says “You never know what might happen,” they don't mean you personally. They mean anyone, everyone, people in general. Similarly, “You can't judge a book by its cover” isn't about you specifically but about what people shouldn't do.

The word shows up everywhere because conversation means talking to someone. Without you, we'd have a hard time giving directions, asking questions, or telling someone what we think. It's one of the most common words in English, quietly doing essential work in nearly every conversation we have.