refers to a traditional server or serverless provider for Server-Rendered Routes, or your build environment for pre-rendered routes. page 
Code that runs on the Elm Backend is co-located with the code for your Elm Frontend, allowing you to seamlessly reuse types and code.
The Elm-Pages Architecture manages the glue to get the data from your Elm Backend to your Elm Frontend giving you a seamless experience of getting data back and forth between the server and the web page through the Route's data (as well as the action for server-rendered Routes).