As a question it means, "Are you an American?"
As a declarative sentence it means, "You're an American."
Eres means "you are". Such as "You are American" = "Eres americano/a"
Tú eres = You are
what are you?
de donde eres = where are you from
you are
Tú eres americano. (using the informal 'tú) Usted es americano. (using the formal 'usted')
It is masculine for "you are mean".
It's you
it mean "only you"
Eres bien mensa means: You are stupid/silly.
"Tu eres mi mundo" in English means "You are my world."
you are pretty