Jesus compares His disciples to salt and light. The two concepts of salt and light seem to illustrate Christ's role in serving as a liaison between God and man, and His disciples' responsibility to follow Him in this work:
Salt is a preservation agent. As the “salt of the earth,” the disciples were to continue Christ's work of representing the earth before the Father in a way that is pleasing. When God sees the earth filled with His righteous saints, His hand of judgment is stayed.
Light allows us to see what is there. The disciples of Christ were to behave in such a way that those who see their good works might respond, as all men were intended to respond, by giving glory to God.