What might a CN filter be in the context of comet watching? Is it showing dust, or gas, or something else?
A CN filter shows abundances of cyanogen, a molecule with the structure $(\mathrm{CN})_2$. Cyanogen emission lines at 387 nm allow us to study gas in the coma of a comet, telling us more about its structure, properties and outgassing. Other gases, like $\mathrm{OH}$, $\mathrm{NH}$ and $\mathrm{C}_2$, also give away a lot of information about a comet, but the often-strong cyanogen lines are a good tool.

The mechanism behind cyanogen production from parent gases isn't entirely clear, although a number of theories have been proposed. For instance, cyanogen could be released by icy grains undergoing sublimation, or by the decomposition of more complex molecules.