In OS X, your keychain contains your saved passwords. This includes all your email accounts in Mail, passwords stored in Safari, and credentials for accessing known Wi-Fi networks. Because it contains valuable secrets, the keychain is encrypted. It can only be opened with your login password.
But there’s a twist. When you log in to OS X, the operating system automatically unlocks your keychain for your convenience. This means that you don’t have to enter your login password every time you want to use your stored passwords.