This header keyword is the "dead time" correction factor. It represents the fraction of the total time per frame that contains useful X-ray data:
The value is between 0 and 1, and depends on the operating mode used for the observation. The value for the standard, full-frame, mode is 0.987337. If an observation uses interleaved mode then there are separate values of DTCOR for the short and long frames.
The observation times entry and the chandra times help file explain how the various exposure keywords are related.