These eyepiece filters either selectively transmit or reduce the transmission of specific parts of the spectrum of light to increase contrast and visibility of deep-sky objects like nebulas and galaxies in light-polluted skies.
Just like colored eyepiece filters, they screw into the base of the eyepiece. Most quality eyepieces have threads in the base of the tube to accept filters. Many manufacturers use the same threading.
The narrowband type of filter isolates one or more spectral lines (a spectral line is a very narrow slice of colored light) or wavelengths of light to transmit while blocking all other light. This is useful when observing objects like planetary nebulas and emission nebulas which emit most of their light in a few specific wavelengths.
Common types of narrowband filters are O III (oxygen-3), hydrogen-alpha, and hydrogen-beta. Each is named for the spectral line it transmits.
The light pollution reduction type of filter reduces the transmission of or blocks bands of the spectrum corresponding to light from manmade sources like mercury-vapor and sodium-vapor lights. They also block light given off from sky glow created by oxygen in the atmosphere. Thus they greatly reduce light pollution while allowing most light from nebulas to pass right on through to the eyepiece. The sky looks darker while extended objects like galaxies are only slightly less bright than with no filter, increasing the contrast between the sky and the galaxy. They are also called ultra-high contrast (UHC) filters because of this contrast-increasing effect on deep-sky objects.