A pronoun replaces a noun in order to avoid repetition.
Subject pronouns are subjects of verbs.
In French, a subject pronoun is immediately or almost immediately followed by its verb. Subject pronouns are labelled by the term 'person', referring to the subject's role in the conversation. Unlike the English pronoun 'I', je is not capitalized unless it begins a sentence. The pronoun tu is singular and, importantly, informal. Use tu to address people your own age and those you know well.