The structure of the spacecraft is the "body" or hull of the spacecraft. It provides a place to mount all of the other subsystems and protects the spacecraft's components from the environment. In the case of manned or otherwise pressurized spacecraft, it provides an air-tight seal and holds the pressurized gas inside the spacecraft. Spacecraft structures typically provide some level of micro-meteoroid and radiation protection for the components and/or crew inside. The structure must be able to withstand the high loads imparted onto a spacecraft during launch.
Wijker, J. Jaap. Spacecraft structures. Springer Science & Business Media, 2008.