Hi,
Here, 'back up' means that there is an obstruction on the road, or perhaps an accident, and a line of stopped or slow cars starts to form. The line of cars has a front and a back, and as more cars join at the back, we say the line of traffic is backing up.
Another example is an obstruction in a drain, where we say the water is backing up in the drain, or even just that the drain itself is backing up.
Best wishes, Clive