Rubble, undergrowth, shallow bogs, steep stairs, low furniture, and many sorts of other impediments are difficult terrain, which hampers movement.
Costs 1 Extra Square: Each square of difficult terrain costs 1 extra square of movement to enter. Some difficult terrain, such as a low wall, is on the line between two squares, rather than in the squares themselves. Entering a square by crossing that line costs 1 extra square of movement.
Because difficult terrain costs that extra square of movement to enter, a creature can’t normally shift into it. However, if a power or some other effect lets a creature shift 2 squares, the creature can shift into a square of difficult terrain.
Large, Huge, and Gargantuan Creatures: If a Large or larger creature enters two or more squares that have different kinds of terrain, count that square of movement according to the most difficult terrain. Count only squares the creature is entering for the first time, not squares it already occupies.
Terrain Walk: Some creatures have a trait called terrain walk, which allows them to ignore difficult terrain in specific kinds of environments. See the “Movement-Related Traits” sidebar for definitions of the different kinds of terrain walk.
Published in Player's Handbook, page(s) 284, Rules Compendium, page(s) 310.