New runners normally ask what is Tempo running? The best answer / explanation I can give has to be the following from American middle distance runner Bernard Lagat.
Tempo running is sustaining a pace near the edge of one’s aerobic capacity, not as hard as racing, in a zone around 90 percent of maximum heart rate.
Bernard Lagat is famous for doing tempo runs of 5000 meters or longer on the trails.
Source Runners World Interview