The phrase "to unlock these secrets" means to gain access or discover hidden knowledge or information that was previously unknown or concealed. Full definition
Along the way, optional targets to be destroyed will pop up that can help unlock secret levels and better endings. (148apps.com)
While treasure chests may contain items or gems, crystals serve one purpose: to unlock a secret level in each world. (thatvideogameblog.com)
They aren't much, and once you've played through them to unlock the secret characters, you'll probably be sticking with the multiplayer battles. (wizarddojo.com)