If you want to find out more about a specific gadget, accessory, or thingy, consult the references listed in each section. They are not a substitute for a clear sky, but they can and do provide improved views of many objects. Furthermore, filters can be of use even in very dark skies, not just ones that are significantly affected by city lights.
Filters work by rejecting light in certain frequency ranges.
As you may already know, many artificial light sources emit over fairly narrow wavelength ranges. All LPRFs are designed to block the wavelength regions associated with these sources. As you might have guessed, there's a catch. These nebulas emit light in only a few wavelengths, predominantly those of oxygen III and hydrogen alpha and beta. Thus LPR filters tend to work well on emission nebulas.