A pronoun is something like ‘you’, ‘me’, ‘her’, ‘they’ and all the other bitty little words that we use to describe things and people without using their real names or descriptions. The English pronouns do sometimes change if they are in different cases: for example, ‘I’ and ‘me’ or ‘he’ and ‘him’. In German (natch) there are a lot more changes because the pronoun always needs to show what gender and case the thing or person is inhabiting right then and there.
The pronouns change according to case and gender like this:
German pronouns in different cases