Devices such as cellphones and wireline telephones require signals to connect from one end to another. These signals are carried on airwaves, which must be sent at designated frequencies to avoid any kind of interference.
The Union government owns all the publicly available assets within the geographical boundaries of the country, which also include airwaves. With the expansion in the number of cellphone, wireline telephone and internet users, the need to provide more space for the signals arises from time to time.
To sell these assets to companies willing to set up the required infrastructure to transport these waves from one end to another, the central government through the DoT auctions these airwaves from time to time.
These airwaves are called spectrum, which is subdivided into bands which have varying frequencies. All these airwaves are sold for a certain period of time, after which their validity lapses.