Often they are a short paved lane meant for slow vehicles to pull off onto to allow slower traffic to pass, but often on mountain roads, it's a gravel off-road area that serves the same purpose. But pulling into one can be a bumpy ride, and the gravel service means it's hard to accelerate quickly until you get fully back on the road so you need to be sure you have a lot of clear space behind you (which can be hard to ensure on twisty mountain roads).

https://youtu.be/sKLhtlO_aZs