In most situations, the game assumes a character is moving over relatively simple terrain. In the Adventuring phase of the game, this assumes well cared for roads or open plains and the Travel Pace Table can be used as normal. Meanwhile, in Combat & Turn Based Action, the game assumes clear dungeon corridors or open ground free of obstructions. However, adventurers often face dense forests, deep swamps, rubble-filled ruins, steep mountains, and ice-covered ground--all of which are considered difficult terrain.
Anything from dense bramble, precarious steps, or tenacious mud can slow you down from your normal movement speed, with every 1 foot of movement you take within difficult terrain costs 1 extra foot. This means you can cover only half the normal distance in a minute, an hour, or a day on the Travel Pace Table in the Adventuring Phase while remain in difficult terrain, or half of your Ancestry's listed Speed in Combat & Turn Based Action. If using a grid map for a combat encounter, each 5 foot square traversed instead counts as 10 feet.
Multiple hazards that would cause difficult terrain in a space do not stack. However, additional conditions that increase the cost of movement (such as crawling while prone) stack with difficult terrain.