Question

what is a line spectrum of hydrogen

Answer 1

An atom emits a photon (particle of light) when it "jumps" from a higher state (state with higher energy) to lower and absorbs a photon when it jumps from a lower state to higher. As the energy of a state is discrete, so are the energies associated with these jumps (or transitions). 

When you excite a lot of hydrogen atoms to go to higher states, they will deecxite by emitting photons of these allowed energies. If you decompose the emitted light, you will see discrete lines instead of a continuous spectrum. Similarly, if you pass white light through a column of hydrogen, the photons having to allowed energies will be absorbed. These are called emission spectrum and absorption spectrum respectively.