The Sun is mostly composed of hydrogen gas. When atoms of hydrogen get excited they emit light at a very specific wavelength, the main one being called hydrogen-alpha or h-alpha for short. The Sun’s inner atmosphere, called the chromosphere is a complex layer where h-alpha light is emitted, absorbed and re-emitted. If you can tune into this very specific wavelength it’s possible to peek at a side of the Sun not normally seen. Here you’ll be able to see glowing atoms of hydrogen picking out complex lines of magnetic force. Sunspot groups look alive with dark fibrils changing on timescales of just seconds. On the edge of the Sun, huge, flame-like prominences extend out for distances often many times greater than the size of the Earth. The h-alpha Sun is a truly amazing and dynamic spectacle.
A hydrogen-alpha filter blocks all wavelengths of light apart from that at 656.281nm. Actually, there is a small range of wavelengths that still get through either side of this value and the width of this range defines a quantity known as the bandpass of the filter. Making the bandpass narrower increases the contrast of surface features making it easier to see intricate fine detail. However, a very narrow bandpass may make it slightly harder to see prominences in their full glory. My solar H-alpha filter has a bandpass of 0.7 Angstroms which is an excellent compromise to supply good surface detail and excellent views of prominences.
Bart Delsaert - Astrophotography 