Oft did the harvest to their sickle yield, (25)
Sure, this line might sound like it's just about farmers harvesting their grain, but did you ever stop and think about what the Grim Reaper (a.k.a. the personification of Death in some Western cultures) is up to? Check out this image. That scythe that the Grim Reaper is holding is also a farm implement, intended for—you guessed it—harvesting the souls of dead people. So it's possible that the sickle these farmers are using is intended to make us think of the grim reaper harvesting souls. Hmm. Sounds a bit morbid, doesn't it?
The paths of glory lead but to the grave. (36)
The speaker wants us to remember that everyone dies. EVERYONE. And all of your worldly ambitions—college, a great career, whatever your "paths of glory" might be—only end in one place: death. Sorry to rain on your parade!
Can storied urn or animated bustBack to its mansion call the fleeting breath? (41-42)
Here the speaker warns against putting too much emphasis on monuments and fancy mausoleums to commemorate yourself or a loved one after death. After all, no fancy-schmancy statue or "urn" is going to bring you back from the dead.