εσύ (singular) and εσείς(plural) are personal pronouns in Nominative Case.
εσένα (singular) and εσάς (plural) are personal pronouns in Accusative Case.
All of them are translated as "you" in English, but in Greek they are used in different structures.
Εσύ αγαπάς τον Γιώργο. = You love George. (εσύ is Nominative, Subject)
Ο Γιώργος αγαπάει εσένα. = George loves you. (εσένα is Accusative, Object)