The system works with two or four flaps that can be angled so they can block the light from hitting where you don't want it to. The barn doors can be fitted to the strobe head. Most I have seen come with a honeycomb-grid, and a set of coloured plastic discs. This way you can make some nice effects on your background. The honeycomb will concentrate the light more precisely than without, and it will stop spilling the light, when being directed through the honeycomb.
The barn door is reasonably inexpensive, but they are also available in different qualities. Surf around. I can suggest Amazon. They have a plenty full selection.