Groundwork is often perceived as a series of exercises (lunging, liberty work etc) performed with the rider on the ground. I believe it is much more important than this. The horseman firstly has to have the basic skills in reading the equine language. They have to have an understanding of the psychological and physical signs coming from the horse. Then the groundwork begins.
Preparing the horse for Riding
Good groundwork prepares the horse to achieve the next level:
to be ridden.
All the maneuvers a horse understands while the person is on the ground can be applied when the rider is in the saddle. If the quality of the horseman’s groundwork is not good, the horse will likely be in trouble when he is ridden.
When achieving basic skills, the rider also learns how and when to ask the horse to execute the desired maneuver.
The rider also learns to read and interpret the horse's answer to the human requests.