Complex sentence: The hydrogen gas follows fluctuations in the underlying cosmic density field--regions where the 21cm emission is more intense correspond to regions where there is more matter--and so the intensity of the emission can be used to reconstruct the power spectrum of matter fluctuations.
Split into simpler sentences: The hydrogen gas follows fluctuations in the underlying cosmic density field. Regions where the 21cm emission is more intense correspond to regions where there is more matter, and so the intensity of the emission can be used to reconstruct the power spectrum of matter fluctuations.