The ‘spam protect’ feature prevents spamming by not allowing the form to be showered with entries (by an automatic/human entity). Once the form filler presses on the ‘submit’ button he/she will be asked to fill in characters in the captcha box. If the characters are not entered correctly the form will not be submitted.
Please note! In Safari/IE6/IE7 or in forms smaller than 220×220 pixels the spam protect prompt (in section 3) will appear in a regular pop up window.
This is what the spam protect window looks like when a user tries to submit the form: