What does D/F VS or G/H SI, 5mm (~.50ct) mean?

D/F means the diamond's color is in the D/F range. The classic "diamond color grading letter" scale starts at D for pure white to K for yellowish. D/F is a very high-quality white grade. SI means the diamond clarity shows Slight (natural) Imperfections when the diamond is viewed under a 10x magnifying glass. SI is a good quality grade for natural imperfections. 5mm is the diameter of the stone, and ~0.50ct means the weight of the diamond is approximately half a carat.