What it is
Hemp is a small, oily seed you cook until it splits. Fish graze it. It holds roach on a river or canal, and it is a classic barbel bed with pellets or meat over the top.
It is not usually the hookbait, except when they get preoccupied — then a single grain of hemp or a tare does the job. Preparing it properly matters: too hard and it's useless; cooked to mush and it clags.