In the scene above, a light is shining on the round pot, but a bit of extra light is spilling onto the column. To remove the spill light (so only the pot is being lit) the right barn door would be pushed a bit to block the spill. Click the "adjust barn door" button to see what happens when the barn door is rotated slightly.

Watch it again, keeping your eyes on the bottom of the column to see the spill light get blocked off.