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
Crawling. While you’re crawling, each foot of movement costs 1 extra foot (2 extra feet in Difficult Terrain).
Climbing. While you’re climbing, each foot of movement costs 1 extra foot (2 extra feet in Difficult Terrain). You ignore this extra cost if you have a Climb Speed and use it to climb. At the GM’s option, climbing a slippery surface or one with few handholds might require a successful DC 15 Strength (Athletics) check.
RollQuest ships the D&D 2024 SRD as a content pack, on a 5e character sheet that does the maths - roll from the sheet and let the numbers work themselves out. Free to start.