You're is defined as you are.
An example of you're is a mother telling her son that he is coming to the store with her.
- I'm awfully glad you're my brother.
- You're welcome to use the pool any time you want.
- Are you sure you're alright?
- I think you still need someone here, but you're definitely getting better.
- I sure hope you're going to deliver what you've been promising all evening.