The dressage scale of training was something that I really struggled with. I am posting these notes to help other children and adults who don't really understand it either. When I was learning it my teacher was not exactly helpful so I ended up having to go to the internet instead. I was extremely surprised by the lack of useful information. I hope that this post proves useful.
Forwardness: A horse is being forward when it is powering itself along with its hind legs not pulling itself along with its front legs.
Rhythm: Rhythm is the term used for the characteristic sequence of footfalls and timing of a pure walk, pure trot and pure canter. (Regularity with energy and tempo)
Suppleness: Suppleness is the flexibility of your horse. (Elasticity and relaxation)
Impulsion: Impulsion is the containment of energy, forwardness or power. The power in the horse's hindquarters to move forward. (Increased energy and thrust)
Contact: Contact refers to the consistent connection the rider has on the horse's mouth through the reins and which the horse should accept at all times. There should be no resistance, no open mouth and no grinding of the bit. It is the rider's job to make sure that the contact remains consistent and elastic, which is achieved through the rider having an independent seat and the horse accepting the rider's leg aids to go forward. (Acceptance of the bit through acceptance of the aids)
Collection: Collection is the increased engagement and activity of the hind legs, with joints bent and supple, stepping forward under the horse's belly. Slowing the horse down a bit. (Increased engagement, lightness of the forehand, self-carriage)
Straightness: A horse is straight when its body is straight. (Improved alignment and balance)