How did the hydrogen gas emission tube help Bohr in his theory of the structure of the atom?
The Rutherford model of the atom was the atomic model that preceded Bohr's. Ernest Rutherford had shown that an atom contained a positively charged nucleus that contained the majority of the mass of the atom. He believed that the nucleus was surrounded by a negatively charged electron cloud. But this model of the atom did not account for certain atomic observances, namely the spectral line emissions of various chemical elements. Line emission spectra of elements means that different elements emit specific wavelengths of light when they are energetically excited with either radiation or heat. This is what is being referred to by the hydrogen gas emission tube: the atomic line spectrum of hydrogen.
Niels Bohr developed his theory of the structure of the atom in 1913. He posited that instead of a nebulous electron cloud, the electrons orbited the nucleus in specific orbits like planets around the Sun in the solar system. These electron orbits were at very specific and fixed energy levels. When an atom gains a specific amount of energy, an electron can jump from a lower energy orbit to a higher energy orbit. When the electron drops back down to the lower energy orbit, the atom emits energy in the form of electromagnetic radiation that is equal to the difference in the energy levels of the atomic orbits. This theory allows for the observance of the line emission spectra as seen in the gas emission tube. Current quantum mechanical theory tells us that this is an over-simplified view of the atom, but it was an important step in our understanding of the structure of the atom.