A subject pronoun is one of those little words that indicate WHO is the subject of the action. It's a subsitute for the name of the person.
Some pronouns depend on the gender of the replaced person.
Another thing not known in english is the concept of being polite. I'm not saying english speaking people can't be polite. But in English there are no special polite or formal pronouns. Let's start by mentioning that in portuguese the subject pronoun is omitted in most cases. This might sound weird, but in portuguese, the verb already contains enough information to figure out WHO is saying it.