Some units can stalk with ease through terrain that is treacherous for others.
Where your unit has the Ignore Terrain ability:
When you move the unit, you may ignore terrain and you never take dangerous terrain tests for the unit. However, you still may not place the unit on impassable terrain.
1. Difference from Epic 40,000: We derived this ability from part of Epic 40,000's rules for Tyranid War Engine units. This allows for more flexibility in unit design.