The lines you have quoted come from Part III of this tremendous poem, and feature the introduction of the bright and vibrant Sir Lancelot and the way that he literally bursts in to the world of shadows that dominates the life of the Lady of Shalott. What is important to focus on is the way that Sir Lancelot is presented. It is the sight of him in her mirror that makes the Lady break the curse and look out of her window, so he has to be impressive enough to make her want to bring down the curse upon herself. In these lines, we can see that he certainly is. Note the way that he is associated with fire and bright light, drawing attention to him and making him an irresistible sight. We are told that the saddle "shone" as if it was "Thick-jeweled," clearly making it attractive, and the helment and the feather "Burned like one burning flame together." What is interesting abotu these images is that they simultaneously make Sir Lancelot an attractive, oustanding figure but also associate him with the metaphor of "burning" to damage and destruction. However innocently, Sir Lancelot is the sight that results in the death of the Lady of Shalott.