umm... Intelligentsia is the Russian term for intellectuals. I dont think Marx wrote anything about this. But Lenin's vanguardism is closely associated with the idea of intellegentsia.
Lenin believed that workers themselves lacked the knowledge required to organize and start a revolution. So in order to launch a successful revolution, the intelligentsias, usually from the bourgeoisie class, need to help workers to develop the revolutionary conscience and organize the revolution, so on and so forth.
Do not say that we have nothing,
We shall be masters of all under heaven!