By default, Hudson 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, Hudson uses captcha if user creates account by themselves. If you'd like to disable CAPTCHA verification, uncheck appropriate checkbox.