The errors routine allows you to check the transforms by converting the instrumental magnitude of each star in a standard field to a standard magnitudes and then subtracting the catalog value for the star, assuming the same band for both the instrumental and standard magnitude. In a perfect world, the mean error would be 0 and the standard deviation would be the same. However, there is always some random scatter in the data. This routine shows you just how much and if there might be a systematic error, e.g., the redder the star the greater the error in a given filter.