In physics, and in particular in biomechanics, the ground reaction force is the force exerted by the ground on a body in contact with it. -Wikipedia
Happens near the top of the swing (P4) for better players and is responsible for changing the direction of the golf club.
Happens when the lead arm of the player is close to a 45* angle and is responsible for getting the club "back in front" of the player.
Happens between P5 and P6 and is primarily responsible for creating the release of the wrist angles during the downswing.
Understanding the Kinetics is great, but understanding the Kinetics and putting them into a proper sequence is key to having a repeatable golf swing. Not only does the swing become more repeatable with a proper Kinetic Sequence, but it also tends to be much more powerful.