Dressage is about showing the judge that you can get your pony to do what you want him to do, when you want him to do it.
You and your pony have to follow a set of instructions laid out in the dressage test.
A judge marks the test. They give you a mark out of ten for how well you carry out each instruction. At the end of the test the marks are added up and the winner is the horse and rider with the highest score.