Isaiah was speaking to a people that had known great destruction and dislocation. He tells them that their lives are going to change. Look back at verses 1-3 in this chapter. The world is changing--deserts will bloom. Into this changed world will come the glory of the Lord. And as the deserts bloom, so do the people.
Isaiah is prophesying healing and transformation, and calling for us to be part of proclaiming that message.
The lectionary has made an interesting choice in pairing this passage with the gospel lesson. In Mark, Jesus ordered everyone not to tell about the healing, but they told it anyway. Isaiah says to tell it.