The visible part of a software
The front-end or frontend of a software is the part that the users can see in the end.
This part of a software is the one that shows the users information. It is therefore the interface between the users and the software. Every interaction that a user makes will be handled by the front-end. The input will be handled by the backend most of the time and the response will be displayed by the frontend again.
The back-end or backend of a software is the part where the logical algorithms happen.