AMBITION SHOULD BE MADE OF SOME STERNER STUFF.MEANS.

Antony is suggesting that if Caesar was ambitious, as Brutus says he was, he would only care about himself, not about the unfortunate. In the previous line, Antony makes the (totally unsubstantiated) claim that "when the poor have died, Caesar hath wept", suggesting that he cared about the poor. He then says that an ambitious person would have been more stern, or uncaring--his ambition would have been made of a sterner substance.