Difficult Terrain halves the Movement Speed of creatures moving through it; that is, a creature must spend 
2 m / 7 ft of movement to move 1 m / 3 ft through an area of difficult terrain.
Examples of difficult terrain include:
- Dense foliage, uneven rocks, certain staircases, and other non-flat areas
- Water and other liquids
- Most types of Surfaces
- Areas created by spells such as
Hunger of Hadar and 
Spike Growth