Try this: https://jsfiddle.net/willwood/amsnh1f8/2/
Copy the HTML code and paste it into any RW styled text or HTML region. Then copy the CSS code and paste it into either the custom CSS box in the Page Inspector or the custom.css file in your theme. Both snippets of code combined should give you an instant shadowed box, almost identical in appearance to the one you showed on the example website.
I’ve added 20px of padding for you, which pushes the content away from the extreme edge of the box. Likewise if you wanted to push the box away from other content on the page, a margin can be used.
The CSS box shadow property is quite complex. But you can find an excellent working example here, and use this free tool to preview and generate the correct code (choose to generate a box shadow):
It is wise to get into the habit of separating HTML code, from CSS code (as I’ve done in my example). It keeps things more simple, cleaner and more readable.
You could also take the same .shadow-box class selector and apply it on an unlimited number of other elements in a page - where you want to apply the same shadow style.