Full Idea
Every substance expresses the whole sequence of the universe in accordance with its own viewpoint or relationship to the rest, so that all are in perfect correspondence with one another.
Gist of Idea
Substances are in harmony, because they each express the one reality in themselves
Source
Gottfried Leibniz (Identity in Substances and True Propositions [1686], p.98)
Book Reference
Leibniz,Gottfried: 'Leibniz Selections', ed/tr. Wiener,Philip P. [Scribners 1951], p.98
A Reaction
Thus 'expression' (something like mapping what is exterior) is the mechanism through which God achieves harmony in the universe. Instants of time are said to be successive moments of perfect harmony.