A half drop pattern is where every other horizontal repeat (from side to side) is dropped down one half of its length i.e. the design repeats itself on the diagonal rather than the horizontal.
Regular horizontal repeat: The pattern repeats across the roll and is positioned at the same place at each selvedge, allowing a straight match for each cut of fabric.
Half drop repeat: The pattern repeats across the roll half way down the vertical repeat, usually to make the design repeats more interesting. This normally means allowing extra fabric for drapes and cutting very carefully as each cut will start at the half repeat.
For more information, please consult a professional interior designer or curtain maker.