Secrets is a preference pane rather than an app, so once you’ve installed it, you’ll find it inside System Preferences, not in your Applications folder.
What is it? Think of it as “System Preferences Plus”. Secrets gives you point-and-click access to hundreds of hidden preferences in OS X and many applications. Without Secrets, the only way of changing these settings is by using a Terminal window and typing stuff like “defaults write com.apple.iTunes hide-ping-dropdown -bool TRUE” (which is the secret setting for hiding the Ping drop-down menu in iTunes.)
So if you’d rather avoid having to mess around with geekery like that, Secrets is your friend. You can browse through all the hidden preferences on your system, or filter them by application. So if you want Mail to always display messages in plain text, or if you want Safari’s tab bar to stay in view even when there’s only one tab open, or if you want to change how often Time Machine does its backups – well, you can change all of those, and loads more, inside Secrets.
It’s free to download, and frequently updated with new items as and when Apple and third party developers push out updates to software packages.
