Knapping is the ability to make things out of rocks, specifically things like flint, obsidian, and quartz (at least in Clok). Knapping can be used to make things like arrowheads or stone knife blades. It is popular among survivalists, as it allows one to quickly make weapons, although they are far less durable than ones made through blacksmithing. It is currently the only way to make arrowheads.
In order to knap, you must be holding an object like a hatchet or hammer, as well as a piece of appropriate stone. Flint can be found by foraging in certain areas, while quartz and obsidian can be found through mining.