The speaker of "To an Athlete Dying Young" by A.E. Housman talks directly to a young athlete, offering advice and reflections on living, dying, and fame. 

In the first stanza, the speaker recalls the time the young athlete, a runner, won a race for his town, and the people were so thrilled with his victory that everyone stood by and cheered as some people hoisted the runner on their shoulders and brought him home. 

Stanza two has many parallels to the first stanza, though this time the race is different. The athlete is now traveling "the road all runners come," as the same men bring him home on their shoulders and set him down, "[t]ownsman of a stiller town." The young athlete is being carried in a coffin and laid to rest in a cemetery, the "stiller town" which the speaker mentioned.

For the rest of the poem, the speaker tells the dead young athlete why he is better off having died young. He calls him a "smart lad" for escaping his earthly life, a place where "glory does not stay," and any earthly rewards for winning wither "quicker than the rose." He assures the young, fallen athlete that he is fortunate because he will never have to see someone beat his record, and he will not miss the cheers of the crowds once the "earth has stopped [his] ears."

Because he is dead, this young athlete will never be one of those 

...lads that wore their honours out,

Runners whom renown outran

And the name died before the man.

So, says the speaker, the young man chose a good time to die, before he can become someone who once won a race--a man who was once famous but has since faded into obscurity. He comforts the boy with the knowledge that he will remain famous among the "strengthless dead, and his crown of laurels will never wither. 

The speaker clearly believes that life is full of such disappointments that dying young is preferable to living out a normal lifespan, and the theme centers around this idea. Because fame fades quickly, even before one dies; the only way to really hold on to fame is to die young. It is a pessimistic view, of course, but it is the view of this speaker (and Housman himself).