Captcha images are the colorful letters and numbers displayed at the end of your form. They are also referred to as a verification number or verification code. Users have to type those characters in a box, to confirm that they are persons, not robots (robots can not read them). Smart Captcha is the code that shows only if your form is being abused (e.g. multiple submissions from the same IP). Smart Captcha will appear only after the second attempt coming from the same IP address.
You will find the smart Captcha option in the Settings → Security section of your form, under Captcha Settings. To add captcha to form, select any of the available Captcha options, as shown below:
You can find the smart Captcha feature in the Form Editor section, also. Just click on the captcha that appears between the form layout and the submit button. You’ll find four options available in the panel to the left, under Visibility: Hide, Smart Captcha, Always Show and Daily Captcha.