This screen allows you to send links to your pictures to your clients. You specify who you want to send the email to and provide the email title, some text before the links and some text after the links. You also have the option to include your own signature.
So when you hit the Proceed button, an email is sent to every address that you've ticked. Naturally each client gets their own email and is unaware who else it has been sent to. The result can be seen with Step 5