The "L" and "N" that these British Columbians are talking about are the equivalent to the graduated licensing programs in many of the states, such as Washington. They also refer to these big magnetic letters that you have to put on the back of your car when you drive to signify that you are a new driver.
When I see them on the road, I think Loser:
Basically, they're a warning to other motorists that the person driving is even worse than the other BC drivers. Beware!