A dropper is the second fly in a two-fly rig. Most often, it's a nymph hanging below a buoyant dry (the dry-dropper or hopper-dropper rig), but it can also be a smaller dry fished off a bigger one or a second nymph below the first.
Tie the dropper's tippet to the bend of the lead fly's hook with a clinch knot. Length: 12 inches for shallow water, 18–24 for moderate depth, 30+ for deep slots. Cast as one rig — both flies go through the air together, both drift together.