In the case of window treatments, the drop is the vertical distance from the top of the rod pocket to the bottom edge of the window treatment.
For valances drop is usually expressed in inches. For curtains the drop may be a specific unit of measure or a category: to the sill, below the sill, to the floor, or puddled on the floor.
The drop you need will involve personal preference and placement of the rod. For a longer, vertical line, place the rod either just below the ceiling line and find a drop that goes all the way to the floor or longer to puddle the curtains. Although, most window treatments are traditionally hung a few inches above the frame. Also remember to consider the header height when considering rod placement.