Captcha images certify that your submitter is a real person and not a robot. You may choose to always show a Captcha image at the bottom of your form, to show a daily changed image or to use Smart Captcha – this will only appear when your form is abused (for example, when multiple submissions come from the same IP).
To enable captcha go to your Form Editor and click on the verification field.
In the left-side panel at Type you can choose one of the following:
The number of submissions made from the same IP can be limited to a certain number per hour, day, week, month, year, and eternity, as well as the total number of submissions received through the online form.