  It is widely agreed that studying the 21 cm emission line from neutral
hydrogen may be our best hope for understanding the creation of the first
structures during the dark ages and the Epoch of Reionization (EoR). This
hyperfine transition occurs as a result of the spin-spin interaction between
the electron and the proton in hydrogen: The energy of the parallel spins
(triplet) state is greater than that of the anti-parallel spins (singlet). This
transition is strongly banned, with a transition probability of $2.6 \times
10^{-15} \mathrm{s}^{-1}$ being exceedingly low. Despite its low chance, the 21
cm hyperfine transition is one of the most important tools in observational
astronomy due to the abundance of hydrogen in the Universe. The relative
occupation number of the ground state and the occupation number of the excited
state determine the strength of the 21 cm emission or absorption. Collisions or
the so-called Wouthuysen-Field effect can both excite the 21 cm line. In this
article, we give a brief review of the basic physics of 21 cm emission and
highlight the current main worldwide 21 cm signal experiments. We also discuss
several related concepts in astronomy and cosmology with these observations.
