Rule
If a space is Difficult Terrain, every foot of movement in that space costs 1 extra foot. For example, moving 5 feet through Difficult Terrain costs 10 feet of movement. Difficult Terrain isn’t cumulative; either a space is Difficult Terrain or it isn’t.
A space is Difficult Terrain if the space contains any of the following or something similar:
• A creature that isn’t Tiny or your ally
• Furniture that is sized for creatures of your size or larger
• Heavy snow, ice, rubble, or undergrowth
• Liquid that’s between shin- and waist-deep
• A narrow opening sized for a creature one size smaller than you
• A slope of 20 degrees or more