The Tell A Friend feature is a A form of eMarketing that gives an incentive to share and refer others to a store. The referrer gets an incentive to refer others, and the store gets more customers. Also known as "Referal Programs" in other forms of eCommerce marketing.
You can add this to your Product Details pages in the HTML editor using the following merge code:
Navigate to Themes > Edit Theme > Product Details > HTML Editor
Now, add this merge code in place. (Check first to see if it's been commented out of the theme by default).
This is what it will look like on the Product Details page.
This is what you will see when the link is clicked.
They will then receive an email that looks like this:
You can alter this under Marketing > Email Templates and find the one titled "Tell A Friend".
For more information on altering email templates check out the article on Email Templates