Hello Sajjad! I agree with teacher William - that it means to overtake in the lane which is considered the "slow" lane. So, for example, in South Africa we have to "keep left and pass right" which means that our "slow lane" is the left hand lane. So if I pass a car that is in the right lane while I am driving in the left lane I would be undertaking it. This is not exactly illegal and people are doing it all the time but it can be very dangerous and could be considered to be reckless driving. I hope that helps!

Thank you David. So British people use this word.

You learn something new everyday. That's not a term I've heard in the US. We would say "passing in the slow lane"

I got it! Thank you so much Teacher William.

It is to go past a car on a motorway/highway in a lane that you are supposed to be going slower than the car you go past.