Here we deal with 1d-spectra in fits-format. These are already results of data reduction processes that have isolated the information contained in CCD images from the spectrum of an object.
The fits-files contain a header with important parameters, e.g. the name of the object, the recording time, the name of the observer and much more. We have already got to know some of the components of the header.
And the fits-file contains the intensities of the pixels added to the spectrum, the flux.