In combat, characters equipped with melee weapons (or creatures with natural attacks dangerous enough to count as weapons) threaten all areas they can reach. Essentially, if a character can make an attack against an opponent in a specified area, they threaten that area. Characters who are off-guard or otherwise incapacitated (flat-footed, paralyzed, held, sleeping, etc.) do not threaten any areas. Generally speaking, most creatures in Pathfinder: Kingmaker are Small or Medium-sized, meaning they’ll threaten a five-foot area around themselves… or thereabouts. Characters with reach weapons (such as longspears) or creatures larger than medium-size may be able to attack targets further away, and hence threaten a greater area.
This can make it tricky for smaller characters to combat larger ones, as larger creatures threaten more area, and can therefore perform Attacks of Opportunity from further away, making it risky to engage and disengage.