If something is secret, it is known about by only a small number of people, and is not told or shown to anyone else.
A secret is a fact that is known by only a small number of people, and is not told to anyone else.
If you say that a particular way of doing things is the secret of achieving something, you mean that it is the best or only way to achieve it.
Something's secrets are the things about it which have never been fully explained.
If you do something in secret, you do it without anyone else knowing.
If you say that someone can keep a secret, you mean that they can be trusted not to tell other people a secret that you have told them.
If you make no secret of something, you tell others about it openly and clearly.