You will often hear horse people talk about a horse's "action." What do we mean by this?
A horse's "action" is the specific way it moves its legs and feet, especially at the trot. Different breeds are often bred to have action that matches their purpose. Racehorses, for example, are bred to have a low, smooth and very efficient "action" whilst carriage and hitch horses have a very elevated action that's designed to, well, look flashy.
If somebody says a horse has "good action" it means they move correctly for their breed and type.