Relative clauses connect two sentences that have something in common. Let’s start with the nominative. Take a look at these two sentences:
The thing they have got in common is Mann. This is what I call the relative. Now let’s put those two sentences together. It takes three steps:
This is it already. It’s that simple. All that is missing is the table of relative pronouns. The blue der in the last example e.g. is called a relative pronoun. Here are all relative pronouns for future reference. Notice the similarities and the differences with the standard definite articles:
This topic is covered in: A2-L46