Peter compares the prophetic word to a lamp that gives light in the dark until light comes in the morning. The coming of morning is a reference to Christ's coming.
Peter speaks of Christ as the "morning star," which indicates that daybreak and the end of darkness is near. Christ will bring light into the hearts of believers, ending all doubt and bringing full understanding of who he is. Alternate translation: "Christ shines his light into your hearts like the morning star shines its light into the world"