Refer to figure.
On a “semi-monocoque” structure, the structural load is primarily carried by the stressed skin, which is reinforced by internal structural members such as formers (frames), stringers, or longerons. The combination of the skin and internal structural elements works together to distribute the applied loads and provide strength and rigidity to the fuselage structure.

On the other hand, a “monocoque” structure relies on the skin itself to carry most of the structural loads without the need for internal structural members. It is a one-piece structure where the skin acts as both the load-bearing structure and the outer covering.