enantiomers are non-superimposable mirror image compounds which are divided into dextrorotatory (Latin: dexter, right) or levorotatory (latin: laevus, left) depending on how a substance rotates the plane of polarization. A racemic mixture is an equal mix of both.
R and S configurations are in relation to the configuration around the chiral, or center, carbon. They are specified as R (latin: rectus, right) or S (latin: sinister, left) {reference Morrison & Boyd's Organic Chemistry 5th edition-yes I still own it} This is done by assigning priorities to the ligands attached to the chiral carbon in the center based on atomic numbers and, I believe, is beyond the scope of the question.
In short, Dexmedetomidine is the dextroisomer of medetomidine (reference-Anesthetic Pharmacology-Evers and Maze Pg 479), just as the name implies. I am unsure of the R vs S configuration of dex.
