This typically means that some suspicious activity was detected from the IP address of the person viewing your website. In order to verify that this is a real person accessing your website, a CAPTCHA challenge is presented. This is when the wiggly letters and numbers are given and you are asked to type in the letters and numbers.
The error that you are describing means that the wrong letters and numbers were typed in. Your website visitor is free to try again, or visit your website from a different Internet connection.