The 21 cm line of neutral hydrogen offers a unique window into the early universe, particularly during the dark ages and the epoch of reionization. This line, resulting from the hyperfine transition of hydrogen, is sensitive to the spin temperature of the intergalactic medium (IGM), which is influenced by various physical processes. Observations of this line can provide insights into the distribution of matter, the formation of the first structures, and the reionization of the IGM. The spin temperature is determined by a balance of processes including the absorption of cosmic microwave background (CMB) photons, collisions with other particles, and scattering of ultraviolet (UV) photons. The 21 cm signal is affected by foregrounds, which are particularly challenging at low frequencies. Despite these challenges, the 21 cm line offers a direct three-dimensional view of structure formation, complementing other observational techniques. Theoretical models and simulations are essential for interpreting the data, as the signals are expected to be buried within foreground noise. The study of the 21 cm line is crucial for understanding the early universe, including the formation of the first stars and galaxies, the evolution of the IGM, and the reionization process. The upcoming experiments, such as the Square Kilometer Array (SKA), aim to provide detailed measurements of the 21 cm signal, which will help constrain cosmological parameters and test theories of structure formation.The 21 cm line of neutral hydrogen offers a unique window into the early universe, particularly during the dark ages and the epoch of reionization. This line, resulting from the hyperfine transition of hydrogen, is sensitive to the spin temperature of the intergalactic medium (IGM), which is influenced by various physical processes. Observations of this line can provide insights into the distribution of matter, the formation of the first structures, and the reionization of the IGM. The spin temperature is determined by a balance of processes including the absorption of cosmic microwave background (CMB) photons, collisions with other particles, and scattering of ultraviolet (UV) photons. The 21 cm signal is affected by foregrounds, which are particularly challenging at low frequencies. Despite these challenges, the 21 cm line offers a direct three-dimensional view of structure formation, complementing other observational techniques. Theoretical models and simulations are essential for interpreting the data, as the signals are expected to be buried within foreground noise. The study of the 21 cm line is crucial for understanding the early universe, including the formation of the first stars and galaxies, the evolution of the IGM, and the reionization process. The upcoming experiments, such as the Square Kilometer Array (SKA), aim to provide detailed measurements of the 21 cm signal, which will help constrain cosmological parameters and test theories of structure formation.