Dictionary.com defines a bright line spectrum as "the spectrum of an incandescent substance appearing on a spectrogram as one or more bright lines against a dark background." Heating a material in its gaseous state causes it to give off light that creates bright lines of different colors when they pass through a prism, according to About.com.
When white light from the sun passes through a prism, it creates the full rainbow of colors. However, the University of Colorado indicates that the heated gases give off light in specific wavelengths that are responsible for the bright line spectra. Each material gives off a spectrum so unique in nature, it allows the scientist to identify the gas he is heating through the science of spectroscopy. In addition to the lines in the visible spectrum, the gases also create lines that are invisible to the human eye but detectable using other devices.
About.com says that as electrons change from one atomic orbital to the next, their energy changes. Changing from an orbital at a higher energy level to a lower one gives off this energy in the form of a photon of energy, which creates the light for the spectra. Since each element has a unique set of electrons in its orbitals, each spectrum is unique.