As your teacher has no doubt explained to you this poem was written by Alfred Lord Tennyson in response to the death of and his subsequent grief for his friend and future brother-in-law Arthur Henry Hallam. While this is not the only work in which Tennyson explored the vagaries of death, it was probably the most enduring. He worked for many years on this memoriam and eventually it took on a life of its own. There are 131 parts to this extensive memoriam. I would suggest that you check out the following e-notes links for a quite extensive explanation of the poem.