A railroad spur line is basically just a branch railroad line that leads off the main railroad tracks and leads into and out of another town such as a small town.
So when the train needs to go into another town or go near someplace such as a scrap yard to load scrap metal onto the train then the train would go down the spur line and deliver the train cars.
The spur line for a railroad is also a train track on which cars are left for loading and unloading and then they are picked up when ready.
Railroad spur lines are also called and known as sidings or side tracks.
When you see the main railroad tracks then see another train track attached to the main train track that leads off to the left or right of it then that's a spur line.