D stands for dense. Dilute is represented by d. As with all genes, the capital letter indicates the dominant gene; the lowercase letter stands for the recessive gene.
A rabbit that is DD or Dd has dense color, as in black or chocolate. A rabbit that is dd has a dilute base color, as in blue or lilac.
If you breed two rabbits that have dilute base colors, they will never produce offspring that are black- or chocolate-based. If you breed two rabbits, at least one of which is DD, those two will never produce blue or lilac offspring.
So far, we have covered the A, B, and D genes. Three more to go.