My limited understanding is that they were in a sort of "overtime" and tied at 5-5. The German fencer had to score a point to win, due to some sort of random drawing before the "overtime". There was 1 second left on the clock, and they had a few stoppages due to them both hitting each other at the same time, but that 1 second never ticked away. At one point the clock hit 0, they added the one second back, and then the German fencer scored a point and won, with 1 second still remaining on the clock.
The dispute is over how long that 1 second actually lasted, I believe.