What does 'Es-tu' mean in English?

It means:

Are you? There is actually supposed to be question mark after that phrase, you can only use it in a question form. - Es-tu un membre d'equipe? (Are you a member of the team?).

It is usually tu es, but when you are asking a question, you reverse the order and put a hyphen in between.

Tu means you

Es means are!

Therefore es - tu? means - are you?

Hope this helped!