Dressage means training - for both horse and rider. I personally believe every discipline should be started in dressage - then once you and horse have the basics down specialize from there. In the lower levels of dressage you are teaching the horse to move forward freely and light rein contact, connected from back to front.
So what does connected from back to front mean? The easier way I can explain it (quickly) is to give examples of a horse being connected and one not connected.
If you ask for a walk to canter transition and horse takes a few trot steps, then horse is not connected. At ALL gaits horse should maintain a steady rythmn with uniform sized steps, and being bringing their hind legs underneath themselves. This way when you're walking and ask for the canter the horse bends it's hocks - hind legs still stepping underneath itself somewhere around the mid belly) and to the rider it feels like the horses shoulders are lifting up and "as smooth as butter" the horse just nicely steps into canter. If the back to front connection is NOT there then horse is not underneath itself and instead some trot steps occur.
When horse is connect - say at walk - it is marching along at a steady rythmn. Rider asks for halt by pressing down (NOT forward but straight down and same weight uniformly) in BOTH stirrups while keeping both legs against horses sides, then rider rocks hips forward pushing horses hind legs into the reins. At that point the rider stops all movement with their elbows (NOT pulling back on the reins) and horse should stop squarely. Horses hind legs should NOT be behind their butt NOR underneath their belly, rather straight down from the hip.
When you and the horse are correctly trained when you move from one gait to another it's like soft butter - VERY smooth and effortless.
Dressage is for Trainers!