Y'all is you plural, or universal you. It's a distinction from you singular, or you as an individual.

It's roughly the equivalent of "youse" or "youse guys" one will hear in and around New Jersey, "yinz" or "you-uns" around the Ohio valley, "you guys" is said all over, and "you lot" from Britain.

That's all it is. You plural. For your example, you likely would have heard it expressed as a super plural, "all of y'all"