A captcha is a way to prevent robots to participate in a automatic and repeted way to your contest.
The captcha is presented in a box where the participant has to introduce the presented numbers. If the numbers are correctly introduced the participant can access the contest, otherwise he is blocked.
With Qualifio you can put a captcha in your contest. You can activate it in the Welcome step of your campaign under the textbox. You can choose between a light or a dark captcha depending on the look&feel of your campaign.