A CAPTCHA is a type of challenge-response test as an attempt to ensure that the response is generated by a person. A common type of CAPTCHA requires the user to tick a box that says "I am not a robot" or to type letters or digits from a distorted image that appears on the screen.
The Form widget comes with a built-in reCAPTCHA, and it helps ensure that your form can't be submitted unless the reCAPTCHA is validated by ticking off the box that says "I am not a robot".
Should a visitor not tick the box, the form post will fail.