Backend code, basically what is ran on the server and manages user requests, database interactions, etc.. Frontend is the user end, so managing input, displaying information from server requests, etc. and is in the form of an app or website page.

As a network guy...open up your favorite web-managed application and open the developer console. Inspect the transactions you see and compare it to the applications REST API reference, and you'll likely find a lot of commonality (and maybe some undocumented endpoints!).

Backend made the API and everything that is performed by it. Front end is doing the GUI based off the response and promoting for input.

Hah. I get it. Good one.