A captcha function presents a user with a challenge that they must complete before they can submit content (usually via a form) on your site. Captchas usually present an image or audio to the user, and require the user to enter text which matches or confirms the content of the image or audio. Captchas are used to prevent or limit automated bots from submitting content on your site.
There are a number of free third-party services you can use to implement captchas on your dotCMS forms. This document provides an example using the free Google reCAPTCHA service, and provides links to several other free captcha service providers which can be integrated into your dotCMS forms.
Google reCAPTCHA uses a public key (called the “sitekey”) and a private key (called just “private”).