This song is one of my favorites from Summoning. From what I understand it is about the Noldor when they left Valinor to go back to Middle-earth. They are marching through Helcaraxe (the Grinding Ice) and the events in Valinor occupy their thoughts. The lyrics seem to be the thoughts of one of the elves which gives us a taste about the uncertainty that many of them had. Some of them had decided to stay behind, others decided to follow Feanor and leave and others decided to follow their kin although they did not whole-heartedly agree to leave. "under endless howling winds" probably signifies the Helcaraxe (the Grinding Ice) which they had to walk to reach Middle-earth. "On and on goes our march gruesome as the Light of sun" can be related to the dying of the Trees of the Valar and the replacement of their light by the Sun and the Moon. I find the lyrics very interesting because the Silmarillion doesn't give us detailed descriptions in many cases and it is written like a history book. We do know that Helcaraxe is a very cold and unfriendly part of Arda and that the march to middle-earth took many years so the song takes us into the situation and it shares possible thoughts that the Noldor had.