By default, Jenkins allows users to create accounts by themselves, via the "sign up" link on the top right shoulder of the page. But if you'd like to prevent random users from creating accounts, and instead tightly control how user accounts are created, uncheck this box.

When this checkbox is unchecked, someone with the administrator role would have to create accounts.

By default, Jenkins does not use captcha verification if the user creates an account by themself. If you'd like to enable captcha verification, install a captcha support plugin, e.g. the Jenkins JCaptcha Plugin.