Full Idea
For individuation, substance needs three properties: independence, to separate it from other things; unity, to call it one thing, rather than an aggregate; and permanence or stability over time. Its other role is as subject for predicates.
Gist of Idea
Substance needs independence, unity, and stability (for individuation); also it is a subject, for predicates
Source
Franklin Perkins (Leibniz: Guide for the Perplexed [2007], 3.1)
Book Reference
Perkins,Franklin: 'Leibniz: Guide for the Perplexed' [Continuum 2007], p.62
A Reaction
Perkins is describing the Aristotelian view, which is taken up by Leibniz. 'Substance' is not a controversial idea, if we see that it only means that the world is full of 'things'. It is an unusual philosopher wholly totally denies that.