Leibnizs metaphysics is illustrated in his Monadology. He states that there are many substances, but only one is infinite (God) the others are finite. He calls simple substances monads. Only the infinite substance can act on any other. Hence any change in a monad must come from within - that is they are windowless - nothing "can enter or depart". Further no monad can act on another.
So to have them act in concert Leibniz outlines his dogma of the pre-established harmony, which is orchestrated by the only infinite substance.
But why must they act in concert? This is something not explained in the monadology - is it to be found in earlier Christian metaphysics?