Das ist der Mann, der im Park joggt.
English: That is the man who jogs in the park.
A relative clause (Relativsatz) in German is a part of a sentence that gives more information about a noun. It uses a special pronoun (Relativpronomen) like 'der', 'die', 'das', etc.
Use German relative clauses to add extra information about a person, thing, or idea mentioned in the main sentence, often to describe or specify which one you mean.
Das ist der Mann, der im Park joggt.
English: That is the man who jogs in the park.
Ich habe ein Buch, das sehr spannend ist.
English: I have a book that is very exciting.
Kennst du die Frau, die dort steht?
English: Do you know the woman who is standing there?
Das ist das Auto, das ich kaufen möchte.
English: That is the car that I want to buy.
Hier sind die Kinder, denen ich helfe.
English: Here are the children whom I help.