In Britain, L-plates are signs with a red 'L' on them which you attach to a car to warn other drivers that you are a learner.
L-plate in British English
noun
British
a white rectangle with an "L" sign fixed to the back and front of a motor vehicle; a red "L" sign is used to show that a driver using it is a learner who has not passed the driving test; a green "L" sign may be displayed by new drivers for up to a year after passing the driving test
Examples of 'L-plate' in a sentence
L-plate
L-plate driver learning to reverse ran over her mother
Times, Sunday Times (2011)
She can rip up her L-plates at last.
The Sun (2013)
But the learner driver managed to break the law as the car was missing an L-plate on the front.