This is the 985th most frequent German word.
bielefeld
"Bielefeld" is a city in northwestern Germany.
Here, 'Bielefeld' is used as a proper noun referring to the name of a city in Germany.
Ich habe letzte Woche Bielefeld besucht.
I visited Bielefeld last week.
Here, 'Bielefeld' is referenced as part of the phrase related to the well-known internet theory about the existence of the city.
Es gibt eine bekannte Theorie, dass Bielefeld gar nicht existiert.
There is a famous theory that Bielefeld doesn't exist at all.
In this context, 'Bielefelder' acts as an adjective derived from the city name, describing something related to Bielefeld.