Full Idea
I understand matter as either secondary or primary. Secondary matter is, indeed, a complete substance, but it is not merely passive; primary matter is merely passive, but it is not a complete substance. So we must add a soul or form...
Gist of Idea
Secondary matter is active and complete; primary matter is passive and incomplete
Source
Gottfried Leibniz (On Nature Itself (De Ipsa Natura) [1698], §12), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 4
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.142
A Reaction
It sounds as if primary matter is redundant, but Garber suggests that secondary matter is just the combination of primary matter with form.