The wording you insert here is displayed to your clients after successful registration as a new client online if they have followed the online "New Client Registration" procedure. Read more about this here in Terms and Conditions.
This wording should typically explain that registration has been successful and provide advice as to what to expect / do next.
Note that this function also supports merge codes, so merge codes entered here will merge the required / appropriate data into the resulting text.
Screen shot - Client Portal Settings
Example of wording displayed after successful online resitration.
Screen shot - Example of Online New Client Registration Process