The input coordinates are pixel numbers along the first and second dimensions of an image, where the centre of the first pixel is located at (1,1) and the spacing between pixel centres is unity.
The output coordinates are right ascension and declination in radians. The celestial coordinate system used (FK4, FK5, etc.) is unspecified, and will usually be indicated by appropriate keywords in a FITS header.
AST A Library for Handling World Coordinate Systems in Astronomy