Primitives are basic geometrical objects that can be used to shape up map geometry along with Static Objects.
The creation window looks like this:
Primitive Type: what type of primitive will be created. At the moment, this EditMode can only create planes.
Material: material file that the object will use.
Tile Amount: the number of repetitions in the texture for a plane sized 1×1
Tile Offset: a vector that will indicate how much the texture should be displaced over the object.
Texture Angle: rotation that will be applied to the 2D texture.
Align to World Coords: if the texture coordinates should align to world. This is useful to avoid seams on planes that are next to each other.
More on primitives here.