The context for this reading is the same as that of Isaiah 7. King Ahaz has been unfaithful to the LORD, and has made shortsighted policies and treaties that would put his country in greater dire straits. To the people struggling with the poor judgment of their king, Isaiah responds with this prophecy. There is a king coming who will rule very unlike King Ahaz. This king will lead the people out of the darkness. He will remove the burdens placed on them. He will be like no other king, not even David. And this king will reign forever.
How has the promised king overcome darkness? What was the response of the ‘darkness?’ For more info on that see John 1:1-13 and John 3:19-21.