A chirality center is a carbon that has four different groups attatched to it. In the case of chloramphenicol, the two chirality centers are on the two carbons that have stereochemistry in the below structure:
You can tell that the two centers are R, because when the alcohol groups coming towards you, the Hydrogens that are attatched to them are going back. You number the groups around the carbon based on priority, and the larger group on the right takes priority in both cases, so you would go around the carbon in a clockwise fashion, which would mean the configuration is R.
Both of these have a configuration of R, as the question stated.
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