Whom and/or what did Moses and Elijah represent in the Transfiguration?

We read in Mtt 17:1-3 (NKJV): > Now after six days Jesus took Peter, James, and John his brother, led them up on a high mountain by themselves; and He was transfigured before them. His face shone like the sun, and His clothes became as white as the light. And behold, Moses and Elijah appeared to them, talking with Him. It is implied that God the Father ordained Moses and Elijah to be part of the Transfiguration, with a special purpose. Moses had been dead and buried while Elijah was thought to be alive( Mtt 27:49). The former was associated with Law while the latter was considered as one of the greatest prophets. My question is : Whom and/or what did Moses and Elijah represent in the Transfiguration ?