Narrowband filters pass a small slice of the spectrum and block the rest, and on the right target the effect is dramatic rather than subtle. Knowing which target is the right one is the whole skill of using them, and it is the part the packaging leaves out.
How they work
Emission nebulae glow at specific wavelengths, mostly doubly ionised oxygen at 496 and 501 nanometres and hydrogen at 656. A filter that passes only those bands removes most of the sky's background light while keeping almost all of the object's, so contrast rises sharply even though the image overall gets dimmer.
UHC and OIII, the two visual filters
A UHC filter passes a band covering both oxygen lines and hydrogen beta and works on most emission nebulae, which makes it the sensible first purchase. An OIII passes a much narrower band and is spectacular on planetary nebulae and supernova remnants like the Veil, and useless on almost everything else. Both dim the field noticeably and both want aperture behind them.
Hydrogen alpha, which is for cameras
The hydrogen line at 656 nanometres is deep red, where the eye is least sensitive, so a visual hydrogen alpha filter shows almost nothing. In front of a camera it is transformative, cutting through moonlight and city skies to record nebulosity no visual filter would reveal. This is a different product from the solar hydrogen alpha systems used to observe the sun.
Questions people ask about narrowband filter
What does a UHC filter do?
It passes the wavelengths emission nebulae glow at and blocks most of the rest, raising contrast on those objects substantially.
Is an OIII filter worth it?
On planetary nebulae and supernova remnants, yes, and the effect on the Veil nebula is dramatic. On galaxies and clusters it does nothing useful.
Can I use a hydrogen alpha filter visually?
Barely. The eye is insensitive at that wavelength. It is an imaging filter, and it is not the same product as a solar hydrogen alpha system.