A catslide is a term used for a roof construction where a section of roof is extended down to cover a lean-to or side room so the roof elevation sweeps down form the ridge often almost to the ground.
The roof continues down below the main eaves height and allows you to have a larger depth of building without increasing your ridge height.
Catslides are often large areas of thatch and many old thatched barns have catslide sections of roof on one side or even on both.
This type of roof construction is not confined to thatched roofs and maybe found on roofs made of other materials eg slates.
The idea of cats sliding from these roofs to the ground being the reason for the name is not one which we can confirm or otherwise we will leave that to you.