A hurdle is a barrier or other obstacle that a runner has to jump over during a race. If you trip on a hurdle, you probably won't win the race.
When they're doing certain types of races, both human and horse athletes have to leap across hurdles (sometimes other animals do this too, like dogs and even rabbits). To do this is also to hurdle. There are physical hurdles, and then there are their figurative counterparts — those difficulties or hurdles that prevent you from progressing in your life.