captcha Module Documentation
A CAPTCHA is a challenge-response test most often placed within web forms to determine whether the user is human. The purpose of CAPTCHA is to block form submissions by spambots, which are automated scripts that post spam content everywhere they can. The CAPTCHA module provides this feature to virtually any user facing web form on a Backdrop site.
Configuration
Go to Configuration > People > CAPTCHA (admin/config/people/captcha). You can either use the default Math CAPTCHA or also enable the included Image CAPTCHA module.
This module allows for additional modules to extend its functionality, such as reCAPTCHA.
Disabling in development or testing environments:
The captcha module can be disabled even if the module is enabled by setting the variable disable_captcha to TRUE. For example, in settings.php you can set
$config['captcha.settings']['disable_captcha'] = TRUE;