To create a 3D map of the Universe we need to measure 3 coordinates of galaxies. The celestial coordinates on the sky are easy to get, but measuring their distance is much harder. Since the Universe is expanding, the photons from a far away galaxy are also expanded during their long journey. The expansion of photons is the redshift: well known spectral features are shifted to redder colors. According to Hubble's law, the distance is approximately proportional to the redshift.
Since galaxies are very faint, it takes a lot of obesrving time to spread their light in spectrographs and get high resolution spectrum. So, we have spectroscopic redshift only for a limited set of them, for others, redshift may be estimated form broadband photometry, the brighness of galaxies took by few color filters. Estimating the redshift from this limited set is called photometric redshift estimation.
Acknowledgements
We thank Professor Istvan Csabai, Ph.D. for providing the description for the challenge.