What “The fault, dear Brutus, is not in our stars, But in ourselves,…” means
The fault, dear Brutus, is not in our stars,
But in ourselves, that we are underlings.
— Cassius, Julius Caesar · William Shakespeare
It isn't fate that keeps us down, Brutus — it's ourselves.
Cassius rejects the idea that destiny rules men's lives: if they are subordinate to Caesar, it is their own failure of will, not the stars. The line is a ringing statement of human agency — though Cassius uses it to talk Brutus into murder.