This is a unit of X-ray intensity evaluated at 5.2 keV, or over a band pass from 2 - 11 keV. If an X-ray source has the same type of spectrum as the Crab Nebula ( Messier 1) between 2 - 11 keV, we can compare them according to their brightness in Crab units. Numerically, 1 Crab equals 1060 microJanskys, and 1 microJansky = 0.242 x 10^-11 ergs cm^-2 sec^-1 keV^-1 or 1.51 x 10^-3 keV cm^-2 sec^-1 keV^-1. Other units used in xray astronomy are 1.0 UHURU count/sec = 1.1 microJanskys; 1.0 OSO-7 Argon count/sec = 17.6 microJanskys; 1.0 Ariel-5 SSI count/sec = 2.5 microJanskys.
Example, at 2-11 keV, the star Algol produces 9 microJanskys or 9/1060 = 0.0085 Crabs, assuming Algol's spectrum has the same shape as the Crab nebula between 2 - 11 keV. We can also convert this into 9/1.1 = 8.2 UHURU counts/sec; 0.5 OSO-7 counts/sec or 3.6 Ariel-5 counts/sec.
This information is from a 1978 review article by Brandt, Doxsey and Jernigan.
No correction is made for interstellar absorption, which can be significant depending on how far away the object is. Algol is much closer than the Crab nebula, but the 'Crab' units include the effects of interstellar extinction on the x-ray flux. The correction can be estimated by knowing the neutral hydrogen column density towards the Crab, and correcting the 'Crab' units to a 'zero-extinction' calibration. This is beyond the scope of this Internet web site!!!