Woolf isn't all mysterious about the title of A Room of One's Own; she really lays it right out there. The point of her essay is that women and all writers need to have rooms of their own. Preferably with locks.

These private rooms give women the ability to think independently and without interruption. Her thesis is that it's the simple practical material things that are most important when you're trying to figure out how to let genius flourish or flow like a river.