According to wikipedia Captcha is a type of challenge-response test used in computing to determine whether or not the user is human. This is an easy and helpful way to prevent form spamming.
Following are steps to add a captcha to your form:
|1-||Open your form in the Form Builder|
|2-||Go to "Properties" panel > "Form" tab > Settings > "On submit" category > "captcha"|
|3-||Turm Captcha checkbox on.
|4-||When your form filler submits the form a small modal window will open asking him/her to prove he is not a robot. Once he turns on the "i am not a robot" checkbox, he will need to pass a small & easy image test.