Astronomical images often contain a so-called World Coordinate System (WCS). The WCS is a prescription for converting pixel position indices (that is, indices into the two-dimensional array representing the image) into physical units. In practice for direct images of a region of sky the WCS will usually transform pixel indices into some celestial coordinate system, such as the Right Ascension and Declination for some equinox and epoch. However, the underlying concept of a WCS is much more general. For example, it is possible to have a WCS which transforms pixel indices into positions in micron on the face of the CCD chip which detected the image. Alternatively, for spectroscopic data one index might be transformed into a wavelength in Ångström. The WCS is stored with the other auxiliary information for the image, such as the instrument and telescope used, the date and time of observation, etc.
GAIA handles all the details of manipulating the WCS automatically (ultimately by using the Starlink AST library). All that you really need to know is that an image might or might not contain a WCS, and an image with a suitable WCS can be annotated and examined in terms of celestial coordinates rather than pixel indices.
The actual way in which the WCS details are stored in the auxiliary
information for an image is rather arcane. There are several
conventions in use for FITS files, none of which are standard. There
are proposals for FITS WCS standards, but these are still under
discussion at the time of writing. If you are interested to find out
more, the AST library is documented in SUN/210
and SUN/211 and there are three papers
describing the FITS WCS proposals (papers
You should be aware, however, that all of these documents contain far more
detail than you need to know in order to use GAIA, and, moreover, they
are not for the faint-hearted.
The GAIA Cookbook