Sometimes you want to emphasize the plural-ness of "you", so you might say "you all". If I was addressing a group of people, I might say:
"How are you all today?" or "It is so nice to see you all!"
From my experience, there really isn't a plural possessive (so, "your all" sounds wrong). Like everybody else is saying, you / your can be both singular and plural, but saying the "you all" adds a bit of extra emphasis that you are talking about the entire group of people.