So, a single-track road is like a really skinny pathway that cars can drive on. Imagine you and your friends are all trying to walk on a tiny path in the forest that only lets one person pass at a time. That's just like a single-track road for cars!
It's usually very narrow, with only one lane for cars to drive in. If two cars meet on the single-track road, they have to be careful and polite to each other so they can both pass without crashing. There are sometimes places called "passing places" where one car can pull over and wait while the other car drives past.
Single-track roads are often found in the countryside because they might be too small to build a big, wide road. They also give drivers a chance to see some beautiful scenery while they drive slowly along the winding road.