Lowercase sie means "she" or "they". (Capitalised when it's the first word of a sentence, of course.)
Uppercase Sie is formal "you". (This word is always capitalised.) This word is taught later in the course.
du and ihr are informal "you" -- singular and plural, respectively. You'd use them when speaking to a friend or a child, for example, while you would use the polite Sie to speak to an adult stranger (or to several of them -- the polite "you" doesn't make a distinction in German between one or several).