The Hydrogen atom is composed of a proton (which serves as the nucleus) and an electron.  Each of these has an intrinsic property called spin.  The energy state of the atom is slightly higher when the spins are aligned compared to when they are not aligned. In the process of going from the excited to ground state the atom emits a photon of 21 cm wavelength or 1420 Mhz.  It is this line that the SRT is designed to detect.  Thus the 21 cm line maps out the Hydrogen in our galaxy.  Since our galaxy is rotating, the spectral line gets Doppler shifted in frequency when we view different regions through our line of sight.  With this information, astronomers can measure the rotation curve of the galaxy and determine its mass as well as see different spiral arms within our galaxy.