Turing Test is an antispam tool that protects you from comment spam, message form spam, and registration spam.
When a visitor wants to leave a comment, or contact someone using your message form, or register for your blog they will see a question that you write. If they don't give the right answer they will see the "reply if wrong" message that you also write. Your question can be as simple as "what color is red?" (answer: red, reply if wrong: no ... think of fire or apples or something that is "red" and you'll get it). You can even use this to make your blog be 'invitation only' by asking "What is the secret code I sent you in email" ;)
You will have to provide your own question, answer, and rejection notice. Turing Test will not work without all three.
Note that this does NOT protect you against trackback spam. Maybe one day, but not today.