Have you sworn not to reveal the information in a thick file of papers? Then you can describe it as top-secret or classified — it is confidential, and must not be publicized or shared.
If you work for the CIA or the FBI some day, you'll likely be responsible for top-secret information. A top-secret mission might mean that you aren't allowed to tell anyone where you're going or what you are doing there. When a government or company classifies something as top-secret, it's either because revealing it could harm people or because it could harm business interests. The recipe for fried chicken at your favorite fast food place? It's top-secret.