As used by HalfRecipe, a drop is the smallest unit of measurement. It is approximately equal to one drop from a liquid dropper, but we use the term for both liquid and solid ingredients.
As used by HalfRecipe, a drop is equal to 1/80 of one teaspoon, or approximately 0.06 ml. Also, 8 drops is about equal to one "pinch."
Because this amount is so small, we wouldn't think poorly of you if you just ignored the drops.
Wikipedia has its own definition for the term drop.