I’ve actually been puzzled by this myself.
I mean, I know the answer, obviously. It’s a politeness plural, patterned after French Vous (and Early Modern English you). And it’s been mainstream in Greek since the 19th century, although Greeks in practice avoid out when they can—because to them it’s much more about distance than respect.
What’s puzzling to me is, it’s kind of early for Byron to be using that form, before any mass influence on Greek of Western European languages, via media or mass education. Certainly no peasant in 1820 talked like that. Maybe the intelligentsia were starting to ape the French that way, and that’s who Byron was talking to.