Subject Pronouns are the "star" of a sentence. They say who does the action of the verb.
1. The "tú" form means "you" and is used to talk to a friend or close relative in informal situations.
2. "Usted" also means "you" but is used in formal situations, like when you want to show respect to an older person or stranger.
3. "Vosotros" is used only in Spain to talk to a group of people informally. In Latin America, "Uds." is always used to talk to a group of people (it doesn't matter if it's formal or informal.)