The person indicates whether the subject is the speaking, the addressed, or the neither speaking nor addressed party. The speaking party (I, we) is called first person, the addressed party (you, thou, you, you) second person, and the neither speaking nor addressed party (he, she, it, and also plural they) third person.
Because the Greek verb forms, apart from the noun forms, already have an indication of the person in them, it is not necessary to indicate the person of the subject in a separate personal pronoun; if the subject is mentioned with a personal pronoun anyway, it usually implies that it has a little more emphasis.

See further at grammar.