Top-secret describes information or documents that must be kept completely hidden — usually from an enemy, and usually by a government or an army. 'Top-secret documents', 'a top-secret military base', 'the plan was top-secret'. In its strict use it is a real security label. Governments sort their secrets into levels — confidential, secret, top secret — and 'top secret' is the highest of them, given to information that would do very serious harm if it got out. Only people with special permission may read it. In that official use it is often written as two words with no hyphen and sometimes in capitals: TOP SECRET. But most people meet the word in its second, playful use: anything you are keeping quiet. A surprise party, a family recipe, who you have a crush on. 'It's top-secret — don't tell anyone.' Here it is a joke borrowed from the serious world, and it sounds light and fun rather than dangerous. Both uses are common; the tone of the sentence tells you which one is meant. It sits before a noun ('a top-secret meeting') or after 'be' ('the recipe is top-secret').