'Thought-out' means planned carefully, with the thinking already done before the doing. It almost never stands alone — it comes with a word in front telling you how carefully: well thought-out, carefully thought-out, badly thought-out, poorly thought-out. If someone just says 'a thought-out plan', it sounds unfinished to an English ear, like a sentence stopping halfway.

So the useful thing to learn is really the whole phrase 'well thought-out'. It is warm but calm praise — the kind of thing a boss writes on a proposal, or a teacher writes on an essay. It says: you saw the problems before they happened. Its opposite, 'badly thought-out' or 'not thought-out', is a common polite way to criticise: it says the idea is not stupid, it is simply unfinished.

It is an adjective, so it usually sits before a noun: 'a well thought-out answer', 'a badly thought-out policy'. After the verb 'be', people usually drop the hyphen and write it as three words: 'The answer was well thought out.' Both spellings are correct in their own place, and hyphens here are loose — you will see 'well-thought-out' with two hyphens too, and no one will mind.