A tempo run can take several forms (cruise intervals, mile repeats, road runs,…) but is one of the more challenging workouts to get right. A well done tempo run requires concentration on your pace, exertion, environment(hills, temperature,…). This run is also one of the most important for a developing XC runner. Tempo runs are a key component of base training, and also maintained through the racing season.
Here are a few links with more information.