When clicking on the "Refer a Friend" link you will be presented with a form to enter your email address, friends email address, subject of email and body. If you are logged into the site then your email address will automatically be populated with the email address from your account. There will be some default text in the subject and body of email but this can be changed.
One you click the "Send Email" button then your friend will receive an email with the data you entered into the form with a link of the page that you was on when you clicked the "Refer a Friend" link.