One of two things, actually.
1. A breed of Warmblood horse that originates in Germany.
2. A fence consisting of a rail over a ditch, often quite frightening for the horse and a test of bravery.
The two terms appear to be a weird coincidence, but this isn't true - both originate from the Trakehnen area of East Prussia.
The Trakehner fence originally referred to fences built in drainage ditches, common in this wetland area.
The breed originated at the Main Stud Trakehnen in the same area.
A kid and a pony jumping a trakehner fence. Image source: Timdnz via Wikimedia Commons.