Scree is bits of broken rock piled up at the base of a cliff. In the mountains, daily heating and freezing of moisture on the solid rock causes it to slowly break the rock apart and tumble down. A scree slope will pile up to its steepest possible angle. Depending on the size and shape of the rock chunks, that angle may be shallow or steep.
Any additional rock that falls on the slope will roll down until it finds a resting place or hits the bottom. The same is true for your foot! When you step onto a slope of loose rock, just like sand, it will give way under the weight until the material under it settles with the new weight. This may be a few millimetres or a few centimetres. In some cases, you may be sliding down the slope along with a couple tons of rock, all looking for stable places to rest.
Established trails across scree tend to be packed indentations where many people have helped push the rock down a bit until there is a slightly wider path across the face of the slope. Scree trails can’t have very steep inclines so they typically run straight across or a slight rise. It is very important to remain on the trail and watch your step.
Screeing is a fun, very dangerous tactic of quickly descending a scree slope. If the scree is small enough and deep enough, you can kind of ski down it in your boots. You are actually creating a mini-rockslide and riding it down. Make sure there are no larger rocks to trip you up and no one below you. And, there’s a good chance you will take a tumble, so doing it only on grape or orange sized scree rather than cantelope and watermelon sized scree is a good idea.