Conrad is trying to show the effect the jungle has on men. The technology on which the Europeans depend so much becomes useless and eventually rots away in the hot, humid jungle environment. The same thing can be said of Kurtz and other colonials who come and try to "civilize" the inhabitants of the African jungle. Their values and usual behavior is useless in this new environment and eventually those value and more behavior "rots" both the Europeans and their technology.
I think technology and "society" are used as symbols for the aspects of our persona that are useful, good and visible. They have their purpose, but need to be taken care of and protected against the constant encroachment of the darker side of us; the jungle.