Frontend is always what the outside or exterior looks like and what users or consumers interact with and are able to see on their own (basically what is made public). Examples of frontend could be a website, interface, mobile app, or even a vehicle HUD. Backend is what is happening in the code, databases, etc. (things that administrators and coders can see). Examples of backend could be databases, servers, a computer bios, or even the computer in a vehicle.
An example of frontend would be your wordpress blog as it is when accessed by typing in the blog URL by you, your peer, or the professor. An example of a backend to that blog would be the administration panel (WP-admin) in wordpress, only you can see access and see this part of your blog, unless you give someone else an account in order to do so. Frontend and backend work hand in hand together to create a seamless experience for the user or consumer. The backend is in control of the frontend and make sure that it is kept working properly. In a more advanced website, the backend would consist of more than just an admin panel. It would include the server, sql databases communicating with each other, and could even include content delivery networks that grab images and content from different servers to save loading time of your website.