Housman, addressing a dead young athlete, tells him that it is better to die at the height of his power and achievement than to fade slowly away through aging and death. He speaks directly to the dead youth saying, "smart lad ... to slip away" from "fields where glory does not stay." Housman uses these lines to illustrate how rapidly the window in which one can be an athletic champion closes. He says, for example, that the laurel wreath given to the winner to wear on his head fades more quickly than roses. Physical abilities decline quickly; in another year or two, someone younger, stronger and faster will win the races this athlete has just won. It's better, essentially, to quit (or die) while you are still ahead, the cheers of the crowd still echoing in your ears as you slip away.
Housman goes on to comment that death provides an oblivion that will spare the athlete the pain of watching his "record cut," in other words, the pain of somebody else outdoing him. This athlete, who is used to hearing people cheer for him, won't have to listen to the "silence" that accompanies a has-been. He won't have to tolerate the indignity of becoming a runner whose "name died before the man," in other words, someone the world has forgotten.
Housman also says to the athlete that, in contrast, in death his glory will not fade. The "strengthless dead," the poet says, will "flock" to see this athlete wearing the laurel crown that, in death, will remain "unwithered."