Like all parts of the Overworld, clouds are solid and can be walked on, forming the ground. However, at least in the Anvil Cloudlands, clouds are more than dirt, they are living plant matter. Every cloudscape is a prairie, a forest, a reef. Clouds are trees, grasses, and bushes that produce fruit and seeds for the animals and humans that live there.
The flying islands and cities are contained by the Galewall, a wall of wind that keeps the floating islands and rocks together. The Cloudlands form layers, much like oil floating on top of water, with different forms of life and adventure at each layer.
Falling is a constant concern in the Cloudlands as it is in many parts of the Overworld. Smart heroes will pick up some safety devices before any serious adventuring in the lands above.