"The Stuff Song" is a duet Finn and Jake perform in the episode "Belly of the Beast" to capture the attention of the partying bears after Finn proposes that if they "bum everyone out" they might be listened to. The mentioned ultimate fate of everything people eat is a metaphor for the bears' situation inside the Monster.
Finn: My hot dog's dead.
| Jake: Oh, um, my doughnut's dead.|
My burger's dead.
My milkshake is dead.
| Finn: All of our favorite foods are totally dead.|
They cannot procreate in little food beds.
We'll eat them up
And turn them into stuff.
| Jake: And we'll cry over their graves,|
But you can't cry enough...
Together: When you miss someone you love,
You can't cry enough.