A secret key is a hashing salt which makes your site harder to hack and access harder to crack by adding random elements to the password.

In simple terms, a secret key is a password with elements that make it harder to generate enough options to break through your security barriers. A password like "password" or "test" is simple and easily broken. A random, unpredictable password such as "88a7da62429ba6ad3cb3c76a09641fc" takes years to come up with the right combination.

For more information on the technical background and breakdown of secret keys and secure passwords, see:

• Ryan Boren - SSL and Cookies in WordPress 2.6
• WordPress Support Forum - HOWTO: Set up secret keys in WordPress 2.6+
• Wikipedia's explanation of Password Cracking
• Lorelle VanFossen - Protect Your Blog With a Solid Password
• Yahoo's Security Password Tips
• Yahoo Security - How to Choose a Password
• LifeHacker - Get high security passwords from Perfect Passwords