On the UK driving test, before you drive off, the DSA examiner will ask you 2 Show Me or Tell Me questions in the car park, one of them might require you to open your car bonnet. These are basic car issues like checking engine oil, how to know the brakes are working properly, checking headlights or using the main beam/fog lights.
If you fail to correctly answer one or both questions on the UK driving test, you will get a driver error.
You CAN NOT however fail the UK driving test just for not knowing the answers to these show and tell me questions, so don't get yourself in a panic if you've just discovered you have to answer these questions, and your test is soon. Although you could fail the test if you clock up more than 15 driving errors, one of which was for Show and Tell.
It is good you familiarise yourself with these questions even after you have passed your test, as they could save you from braking the law (tyres), having a car breakdown on you or even be involved in an accident.
Show Me Tell Me De-mist Windscreen Video
UK Driving Test Driving Instructor DSA Show Tell