The well-known 21cm line (hydrogen line or HI line) is hydrogen's 21.1 cm (microwave) spectral line, which comes from the difference between two neutral hydrogen ground states (hyperfine structure). It is a useful sign of the presence of neutral hydrogen. It is a forbidden line, i.e., virtually never created under everyday Earth-bound conditions.
It is recognizable at a lot of redshifts, and passes through dust clouds better than much electromagnetic radiation. HI region regions in galaxies produce 21cm emission lines used to estimate galaxy mass, and map out velocities through analysis of Doppler shifts. Knowledge of the Milky Way's structure owes much to this analysis. Regions in front of hot objects, i.e., with a continuous spectrum yield 21cm absorption lines, also useful for analysis.
The neutral hydrogen existing between recombination and the epoch of reionization is presumed to produce 21cm emission lines, which could reveal much about reionization and times since, including analysis of gravitational lensing to reveal galaxy clusters and Doppler shifts that would reveal voids. The signal is weak and has yet to be detected (as of 2017) but much effort has been put into detecting it (21cm experiments), such as PAPER and HERA.
The analogous line for deuterium is 91.6 cm (aka 92cm line). In sources sufficiently bright to see it, this allows determination of a D/1H ratio.