Frontend is what is being actually interacted with by the intended user.

In a car the frontend is:

• Steering wheel
• Gear stick
• Wheels
• Seats
• Pedals
• Lamps
• Ports for pouring in oil, windscreen washer, fuel
• All the different buttons on the dashboard and elsewhere
• Doors
• Keyholes

In a car the backend is:

• Engine
• Gearbox
• Fuel pump
• Cabling
• The actual lightbulbs inside the lamps
• Mechanism for reacting to key being turned in the lock

Etc.

Frontend is what is what is being physically used, backend is what actually makes all of this function. Of course anyone can also go poke at the backend, but without some very deep knowledge, you’re bound to break something. Better let the backend specialists (regarding cars: a mechanic) handle whatever counts as backend if they need to be touched.

In a physical shop the frontend is anything the customer interacts with, while backend includes for example the workers’ pause room, the intermediate storage for goods brought to the store by lorries, etc.