A barndoore is simply a shadowbox
which fits around the "display" to create a dark area around it. You've probably done this with your hand cupping it around the camera display to block out light and make it easier to see the image.
You can make one that will not stand the test of time, or you can buy one that might.
Look at this picture -- rather an extreme example, but once you know what the thing does, you can figure out some different ways of building one.
All of the old 4 x 5 press cameras had barndoors around the viewing screen.
Hope that helps.