Quantum mechanics tells us that any atom or molecule will emit photons (packets of light) at a particular set of frequencies under a particular set of conditions. Any one of these special frequencies is called a "transition," and the frequency (or wavelength) where this transition can be observed is predictable from Quantum Mechanics.
A spectrometer is like a very sophishticated prism, used for measuring the intensity of the photon flux at different frequencies. When humans view optical light, these different "frequencies" correspond to different perceived colors.
When an atom or molecule is moving as it emits a photon, the frequency of that photon will be changed by an amount proportional to the atom or molecule's velocity with respect to the observer. This is called the Doppler shift.
So, if one measures transitions coming from gas particles (atoms or molecules) which are moving, one will measure a peak (see figure at right) centered on the "rest frequency" of the transition, and with a width characteristic of the distribution of velocities in the emitting gas. In the illustration here, the x-axis is labelled in velocity units, using a simple Doppler conversion of frequency (measured by the spectrometer) to velocity (at the source). In the example, "250" corresponds to the rest frequency of the transition.
The color bar in the Figure shows what a "prism" produces as a spectrum at optical wavelengths. The x-y version of the spectrum above the color bar essentially shows the same thing: intensity of emission as a function of frequency.
