The basic idea of this famous poem is that it is better to die young, in your full strength and with all your abilities, rather than to grow old and gradually fade away before dying. The poem is addressed to an athlete who has died young, who is therefore, according to the speaker, a "smart lad" to leave earth, where glory fades very quickly. Note what the speaker says about why he is better off dead at this point in his life:
Now you will not swell the rout
Of lads that wore their honours out,
Runners whom renown outran
And the name died before the man.
To the speaker, there is no worse fate than to lose your fame and to have your "name" die before you. It is far better to die in the full flourish of your fame, so that you will be remembered forever, than to live a long life but gradually be forgotten until nobody will even remember your name. Fame is described as a "garland briefer than a girls," which explores the transitory nature of fame and being a person of renown. The poem insists that death is worth it if that death allows a kind of immortality.