This feature allows to auto-create User in WordPress after doing SSO with the user credentials if the user is not already present in WordPress User list
(username has been purposefully mapped wrong, for it to stand out)
This is the default behaviour of the plugin, out of the box, you do not need to configure any setting to make this work.
You can turn this off to allow only existing users to SSO into your WordPress website.
You can do so by turning off (unchecking) the option “Auto register Users” in the Sign In Settings Tab -> Advanced Settings