The first is understanding their cool outer layers, or atmospheres. 2 O, CH 4 and also solid dust particles. The second is a full understanding of their interiors. The interior densities and pressures are such as to range from an electron degenerate gas in the centre to a near perfect gas in their atmospheres. This encompasses a combination of pressure and temperature that is much more complicated than that found in normal stars and also beyond the range that can be studied in the laboratory.
Brown dwarfs are important in another way.
And there are many more still waiting to be found. If we have only found one third of the nearest M dwarfs, then it is not surprising that it has taken so long to find a brown dwarf in the solar neighbourhood. Indeed this problem of dark matter extends beyond galaxies to clusters of galaxies and the Universe as a whole. Brown dwarfs have no nuclear furnace to sustain them and become very faint as they age, so they are a potential reservoir of dark matter. They could make an important contribution at least to the dark matter in galaxies.