In Leibniz's philosophy, a monad is a conscious unit of perception. You could say we are all monads insofar as we are individuals, or undivided. On some essential level, we do not interact; we live in our separate universes and send signals to each other which we somehow receive and interpret. The most interesting feature of Leibniz's cosmology is how he deals with this problem of how a multiplicity of independent units of eternal substance, the monads, can find common markers so as to learn to coexist. His idea is that there is something called "preestablished harmony."
This is what lets my monad talk to your monad. And any soul communion is posited upon music, essentially. The Leibnizian parallelism of substances reflects what Mozart heard when he wrote the Jupiter Symphony.