Two stacked polarizing filters create a variable-density gray filter, useful for observing the Moon. This entry represents the variable filter's darkest setting.
To use the variable density filter, thread one polarizer into your eyepiece and thread the other into the adapter shown in the photo. Place the adapter into the telescope, then put the eyepiece inside as if into a Barlow lens. By turning the eyepiece in the adapter you can adjust the image brightness to a level that suits you. The adapter does not support the weight of the eyepiece very well, so be careful that it does not fall out.