Horses are often referred to as being hot-blooded or cold-blooded. What does this mean? To some extent it refers to their breeding. The Arabian is the quintessential hot-blooded horse, a term that perhaps refers to its desert origins, but also to its very active, energetic nature. Draft horses are called cold-bloods, as they originate in cold northern climates and tend to be more placid and slow moving. Many horses have lineages that trace back to multiple origins, combining a bit of Arabian, some Thoroughbred, and northern Draft or Pony breeds. These are commonly called warmbloods.
Any horse can be considered "hot" of course. Some Arabians and Thoroughbreds are quite gentle and placid. Some draft horses are quite flighty and skittish. In a behavioral sense, I think, "hot" means that the horse is more likely to react to a frightening situation by trying to fight or flee instantly. Other horses, sometimes considered "stubborn" react to fear by shutting down and standing still, refusing to move when pressured. Both behaviors can be successful reactions, in terms of preserving the horse's safety.
As an example, a friend has a dog which sometimes gets excited and runs into the paddock to chase the horses. Several of the horses respond by running away, then turning and charging the dog. But the donkey and pony in that paddock have a different strategy. They move to one corner of the paddock, where they are protected on two sides by the fence, and then stand very still. The dog loses interest in them and pursues the other horses instead. Such "stubborness" would also be beneficial in dangerous terrain or footing, where fleeing might cause them to slip and fall.
Both reactions are best solved by the rider through gentle, patient training, as an angry reaction only increases the fear of the horse.