It could be the beginning of a sentence concerning a female person.
du bist eine ... - you are a ...
It can also be an equivalent of the following expressions, said in resignation or exasperation to a female person, meaning:
Du bist mir eine feine Freundin! - A fine friend you are!
Na, du bist mir eine Expertin! - Some expert you are!
It can also be said in admiration, meaning the exact opposite.
You are beautiful
bist du in Ordnung = are you okay
Du bist eine hübsche Frau, du bist süß. = You're a pretty woman, you're sweet.
du bist scharf = you are sharp
You are great
"Du bist alles" means "You are everything".
Du bist die Ruh' = you are the peace.
Wie alt bist Du? means: how old are you?
You are I.
"You are there?"
Du bist meine Liebste. - You are my beloved. Du bist meine liebste Oma. - You are my dearest grandma./You are my favourite grandma. Du bist mir die Liebste. - You are most dear to me./You are my favourite.
Du bist der absolute Hammer! = You are totally awesome!