Translation:Are you happy?
"Eres" (you are) is the familiar second person singular form of "ser." "Ser" is used for permanent states, conditions, characteristics, identities, professions, etc. "Estás" (you are) is the formal second person singular form of "estar." "Estar" is used for time, weather, location, changing conditions, etc. In Spanish, there are many different ways of saying "you." In English, the one word "you" is used for singular and plural, subject and object.
Because the person is being addressed informally, with the tú form. There are five ways you could translate the English sentence:
- ¿(Tú) estás feliz? - Are you happy? (informal)
- ¿(Usted) está feliz? - Are you happy? (formal)
- ¿(Vosotros) estáis felices? - Are you happy? (plural, informal, not in LatAm)
- ¿(Ustedes) están felices? - Are you happy? (plural)
- ¿(Vos) estás feliz? - Are you happy? (singular, informal, in parts of LatAm)
Spanish is a so-called "pro-drop" language, which means that subject pronouns are usually left out of sentences. For this specific sentence that means that since the form estás already tells us that it's "you" who feels happy, the actual tú will be left out in most circumstances.