Interpreters use relay interpreting when the language spoken in the room can not be translated by the interpreter him-/herself. She or he connects to another translation booth where the language in question is covered and is translated to a language that is understandable for her-/himself.
In a meeting room where Polish is covered in the German translator, all other booths will listen to the German translation of the Polish spoken words and translate the German to their own language. But German is already a translation from another language (Polish), that's the reason for the name 'relay translation'.
The used interpreter for further interpretation is called a 'pivot interpreter'. The role of the pivot interpreter is thus more difficult because the quality of the other 'relayed' interpretations depends on his or her performance.
When relay interpreting is used, the time for communication increases as does the possibility of losing some information on the way.