Dangerous Terrain covers things like forests, rivers, marshes, etc.

When a model marches, charges, flees, overruns or pursues into or through an area of dangerous terrain, it is called upon to take a Dangerous Terrain test – roll a D6. If a 1 is rolled, the model suffers a wound with no armour saves allowed.

Cavalry, Monstrous Cavalry, Chariots and Shrines treat all terrain other than open ground as being dangerous terrain, as described above. a Chariot or Shrine that fails a Dangerous Terrain test suffers D6 Wounds instead of 1.