"Front-end" typically means the parts of the project a user interacts with--such as the graphical user interface or command line. It's a vague term, there isn't an exact definition.
"Back-end" means the parts that do the work, but the user is unaware of or cannot see. Databases, services, etc.
Think of it like a restaurant where you can't see the kitchen. As a customer you see the front-end--the decorations, menus, wait-staff. Meanwhile the kitchen and stockroom are out of view, but preparing food.
