Smeed's law is a rule about how traffic behaves on the road. Let's imagine you are playing with toy cars. When you have too many cars trying to go down the same road, they get all jammed up and stuck. This is like when there is too much traffic on the road, and everyone has to stop and wait. This can cause accidents and make everyone late!
Smeed's law says that as a road gets busier with more cars, the speed they can travel decreases. So, if you have a small number of cars on the road, they can go fast because there isn't much traffic to slow them down. But, if there are lots of cars on the road, they have to go slower because there isn't as much space to move around.
This is why sometimes you might be driving on a quiet, empty road and can go very fast. But when you get to a busy area with lots of cars, you have to slow down because there are too many other cars around you.
So, in summary, Smeed's law says that the more cars there are on a road, the slower they have to go to avoid accidents and keep everyone safe.