The poll is the point at the top of a horse head, just behind the ears, where the skull meets the first neck vertebra. It is a sensitive, flexible junction central to how a horse carries its head and yields to the bridle; pressure here from a poorly fitted crownpiece or a harsh hand is a common and often-missed source of resistance.
See also: the forelock that drapes over the poll, paired landmark at the other end of the topline
Further reading: Poll (livestock anatomy) on Wikipedia; physical characteristics of horses from the Merck Veterinary Manual.