A secret is something you don’t want everybody to know, like your secret crush on the bus driver. A secret can also be a mystery known to only a few — the secret of the lost pyramid.
There might be a secret passageway in your basement that only you know about. You could have a secret passion for Japanese anime comics and share that secret with only your closest friends. If you're "in on a secret" — plans for a surprise birthday party, for instance — that means you're in the inner circle of people in the know. Everybody has to keep the secret, or the party won't be a surprise.