Static Mesh assets are the basic unit used to create world geometry for levels created in the Unreal Engine. These 3D models are created in external modeling applications (such as 3dsMax, Maya, Blender, and so on) that are imported into the Unreal Editor through the Content Browser. The vast majority of any level made with Unreal Engine will consist of Static Meshes, generally in the form of Static Mesh Actors.
Since Static Meshes are cached in video memory, they can be translated, rotated, and scaled and can be more complex than other types of geometry.