Adding description in registration form

If you're using the JFBConnect registration flow, there are a few language strings in the /language/en_GB/en_gb.com_jfbconnect.ini file (or whatever language file your site would use). You can edit all of those strings there. The section you're looking for looks like:

; Registration - Normal Registration messages
; Instructions
COM_JFBCONNECT_WELCOME="Welcome"
COM_JFBCONNECT_THANKS_FOR_SIGNING_IN="Thanks for signing in using %s. Do you already have an account on this site, or would you like to create one?"
COM_JFBCONNECT_THANKS_FOR_SIGNING_IN_REGISTER_ONLY="Thanks for signing in using %s. Please fill out the information below to create your account."
COM_JFBCONNECT_EXISTING_USER_REGISTRATION="Login"
COM_JFBCONNECT_EXISTING_USER_INSTRUCTIONS="If you already have an account, just log in here, and we'll link your %s account to it."

I hope that helps,
Alex

The page you're looking at is the standard Joomla registration page. That isn't generated by JFBConnect. We don't have language files for that page, and I'm not sure what is adding that value on that page. It's not from JFBConnect though.

To modify the fields on that page, you'd need to use the standard Joomla language files or the Joomla Language Manager, which lets you create overrides for those fields.

On the second page, I'm not sure what extra message you're talking about that you don't like. Either way, that, again, is the standard Joomla login page and not generated by JFBConnect.

I hope that helps explain,
Alex