a prefab is a prebuilt structure that you can place in game at a certain cost. The cost is determined by how many blocks the prefab uses and what materials it uses

how would i lay down a prefab

In the main menu choose the class editor to pick your 10 prefabs - make your own, use the ones we provide to subscribe to some in steam workshop.

Press B to go to build Mode and then Q to toggle between blocks and prefabs. The mouse wheel cycles the prefabs click the mouse to start building