The invisible part of a software
The back-end or backend of a software is the part where the logical algorithms happen.
This part of a software is the one that handles requests and makes complex information handling and processing possible. A good sample of backend development are the databases and everything that is happening on a server far away from the end users - so everything that is very important for the system to run properly but is it not shown to the users.
The front-end or frontend of a software is the part that the users can see in the end.