Broadly speaking, short words are best, and the old words, when short, are best of all.

Churchill is advocating a plain style: prefer brevity (“short words”) and, among them, the familiar vocabulary of long use (“old words”). The point is not antiquarianism but clarity and force. Short, time-tested words tend to be widely understood, rhythmically punchy, and less prone to abstraction or jargon. In political and public writing—where Churchill excelled—such diction helps ideas land quickly and memorably, and it can carry emotional weight without ornament. The remark also implies a democratic ideal of language: effective prose should meet readers where they are, relying on common speech rather than technical or fashionable terms.