Specifically, it refers to the city of Trier (pronounced "Tree-er" in German) in Germany.
Therefore, someone with the last name Trier likely has ancestry from or lived near the city of Trier.
Here's some additional information:
* Trier is the oldest city in Germany, with a rich Roman history.
* It's located in the state of Rhineland-Palatinate, near the border with Luxembourg.
* The city is known for its Roman ruins, including the Porta Nigra (Black Gate) and the Amphitheater.
So, someone with the last name Trier may have a family history connected to this historical city.