  The dark age of the universe, when no luminous object had existed, ended with
the birth of the first stars, galaxies, and blackholes. This epoch is called
cosmic dawn. Cosmic reionization is the major transition of the intergalactic
medium (IGM) in the universe driven by ionizing photons emitted from luminous
objects. Although the epoch through the dark age to reionization is a milestone
in the universe, our knowledge of this epoch has not been sufficient yet.
Cosmic 21cm signal, which is emitted from neutral hydrogen, is expected to open
a new window for this epoch. In this review paper, we first introduce the basic
physics of the 21cm line and how first stars impact on the 21cm line signal.
Next, we briefly summarize how we extract astrophysical information from the
21cm line signal by means of statistical and machine learning approaches. We
also discuss the synergy between the 21cm line signal and other emission lines.
Finally, we summarize the current status of 21cm experiments.
