Spectral Line Emissions from Atoms and Molecules
While the mechanism behind thermal-related energy emissions from ionized gases involves electrons becoming detached from atoms, line emissions from neutral hydrogen and other atoms and molecules involves the electrons changing energy states within the atom, emitting a photon of energy at a wavelength characteristic of that atom. Thus, this radiation mechanism is called line emission, since the wavelength of each atom occupies a discrete “line” on the electromagnetic spectrum.
In the case of neutral (not ionized) hydrogen atoms, in their lower energy (ground) state, the proton and the electron spin in opposite directions. If the hydrogen atom acquires a slight amount of energy by colliding with another atom or electron, the spins of the proton and electron in the hydrogen atom can align, leaving the atom in a slightly excited state. If the atom then loses that amount of energy, it returns to its ground state. The amount of energy lost is that associated with a photon of 21.11 cm wavelength (frequency 1428 MHz).
Formation of the 21-cm Line of Neutral Hydrogen

Hydrogen is the key element in the universe. Since it is the main constituent of interstellar gas, we often characterize a region of interstellar space as to whether its hydrogen is neutral, in which case we call it an H I region, or ionized, in which case we call it an H II region.
Some researchers involved in the search for extra-terrestrial intelligence  have reasoned that another intelligent species might use this universal 21-cm wavelength line emission by neutral hydrogen to encode a message; thus these searchers have tuned their antennas specifically to detect modulations to this wavelength. But, perhaps more usefully, observations of this wavelength have given us much information about the interstellar medium and locations and extent of cold interstellar gas.