Climbing

Source: D&D 5.2 SRD Rules Glossary

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.