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Hi Jimmy,
The numbers refer to the % transmission of particular wavelengths of light. Wavelengths of light are in the nanometer range (very short). We see these wavelengths as the different colour of the spectrum (rainbow).
The oxygen/hydrogen bit refers to the fact that when the gases are stimulated by electromagnetic radiation, like heat or light or ultraviolet, etc, radiation, gases, here Oxygen & Hydrogen, in turn 'glow' at specific wavelengths. Different gas, different glow colour.
These filters allow these specific wavelengths of light to be transmitted, blocking out pretty much everything else. This way the sky-glow is reduced to zero, and everything else the same except for those specific wavelengths. That is why any stars that are visible through the filter appear greenish, and nebulae are able to then stand out.
So, the OIII filter transmitts:
* 93% of 496 nm wavelength, specific to Oxygen
* 93% of 501 nm wavelength, specific to Oxygen
* 40% of 486 nm wavelength, specific to Hydrogen.
You would use this filter to help you better see nebulae like M42 (Orion nebula), eta Carina neb., etc.
These filters are NO GOOD for galaxies as galaxies glow across the entire light specturm
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