An absorption line is a spectral line due to absorption of a specific wavelength within a continuous spectrum of electromagnetic radiation that is passing through a transparent medium such as a cloud or atmosphere. The lines occur due to a preference of atoms to absorb radiation at specific frequencies that match the energy level of transitions to an increased level of atomic excitation. The lines reveal information both about the makeup of the gas and its temperature.
Per Kirchhoff's laws, absorption lines indicate a cooler gas in front of a hotter source of radiation, presumably an approximate black body. One case of such "cooler gas" is the photosphere of stars, and absorption lines are the basis for stellar spectral classes.
By contrast, emission lines are produced when a thin gas has sufficient atomic excitation that relaxation to lower levels is a significant source of its radiation.
For some phenomena, both absorption and emission lines are observed for an element, e.g., for supernovae, if some gas is blown away initially sufficiently energetic to produce emission lines, and later cooler and in front of a hotter material closer to the supernova center.
Telluric lines are absorption lines from the Earth's atmosphere, and are a challenge to carrying out spectrography on the ground (telluric contamination).