a brown horse
a brown horse = a horse + brownness
a scholar = a human + education
This may seem wrong, since when Socrates dies, his matter persists, although he no longer exists.
But: when we are tracing the history of Socrates through time, we do not follow the course of the matter that happens to compose his body at any given moment, but that of the form that the matter has. (Animals and plants metabolize; the matter that they are composed of differs from time to time.)
So what makes Socrates the kind of thing he is, and what makes him remain, over time, the same thing of that kind, is the form that he continues to have.
For Aristotle, the form of a compound substance is essential to it; its matter is accidental. (Socrates could have been composed of different matter from that of which he is actually composed.) c
Form may be accidental to the matter that it informs, but it is essential to the compound substance (i.e., the compound of matter and form) that it is the form of. Form is what makes the individual plants and animals what they are. Therefore, it is the substance of those individuals.