Scree is like a bunch of little rocks that have fallen down a big hill or mountain. Imagine if you took a bunch of pebbles and threw them down a slope - that's kind of what scree looks like! Sometimes scree can be small and pebbly, like if you took a bag of marbles and threw them down a hill. Other times it can be bigger and rockier, like if you took a bag of big handfuls of gravel and threw them down a steep slope.
Now, why does scree happen? Well, when you have a big mountain or hill, sometimes the rocks that make it up can get loose and start to tumble down. Maybe it's because of erosion, which is when wind or rain wears away the earth that holds the rocks in place. Or it could be earthquakes or other forces that make the ground shake and jostle the rocks loose.
Either way, once a rock starts tumbling down the mountain, it can gather up more rocks and stones as it goes. This makes the rock bigger and heavier, and it keeps tumbling until it reaches the bottom of the hill or mountain. Over time, all of these tumbled rocks can start to pile up and form what we call scree.
Scree isn't always fun to walk on, since the rocks can be unstable and slide around under your feet. But it's a pretty cool natural phenomenon, and it can make for some awesome views when you're out exploring the outdoors!