fundamental astronomy - IDL (particularly GBTIDL) velocity axis explanation?

I have been working with GBTIDL (the Green Bank Telescope version of IDL I suppose) to look at some spectra, and in addition to having the x-axis be frequency or wavelength, you can also have it in velocity. I'm a little confused on how this works, because we only have the observed frequency from the spectra, and we would need a rest frequency to find out what velocity the object is moving at, so how can it give me a velocity axis without that information?

Thank you.