This shows some of the detail of a very elaborate altarpiece used to hold a relic, in this case a drop of blood said to be Jesus'.  The scene shows the Last Supper.  Prominent in the foreground is Judas (you can see the purse hanging from his belt).
The side wings are hinged and they can be swung closed.  This suggests that its normal position is closed, and that it would have been opened for use, probably on special occasions such as feast days.