In Spanish, you can say who is speaking or to whom somebody is speaking about by the form of the conjugated verb. This is different from English where most of the forms of the verbs are the same: I eat, you eat, we eat, they eat,etc.
So in English we have to use the subject pronouns all of the time. But in Spanish, since each form is unique and reveals the subject, we don't have to use the subject pronouns. You may use a subject pronoun or even a name so it is clear about whom you are speaking.