Jesus tells Peter that when he was younger he girded himself and went where he wished, but when he is old he will stretch out his hands and another will gird him and carry him where he does not wish to go.
Jesus in Context
Stretch Forth Your Hands: Peter's Coming Death
Jesus contrasts Peter's former freedom with a future in which others will control his movement, and John explicitly says the saying indicated the kind of death by which Peter would glorify God.
Introduction
Historical and Cultural Context
John immediately interprets the saying for the reader: Jesus said this to signify by what kind of death Peter would glorify God. "Stretch out your hands" has often been connected with crucifixion, and early Christian tradition associates Peter's death with crucifixion in Rome. John's wording itself, however, should be allowed to set the minimum claim: Peter will eventually lose his freedom and die in a manner that bears witness to God.
The contrast with Peter girding himself is deliberate. Earlier in the same chapter Peter girds himself before entering the water; Jesus now speaks of a future when another will gird him.
Why It Matters
John prevents readers from treating the saying merely as a metaphor for aging. He explicitly identifies death as its subject. Peter, who once feared association with Jesus, is told that his future faithfulness will ultimately extend even to death.