Jesus had a talk with Peter, but then Peter asked Jesus a question regarding another disciple. In response to Peter's question, Jesus asked Peter that if Jesus "will[ed] that he remain till [Jesus] come[s], what is that to [Peter]" (21:22). Then Jesus told Peter to follow Jesus (21:22).
With Jesus's comments, Jesus wanted Peter to focused on Jesus's plan regarding PETER. From the dialogue, we know that Jesus wants us to focus on Jesus's plan regarding US. God gave us a calling; maybe it's a good idea to know ours.
There is talk about how one of the disciples would not die, but that is actually not the case. Jesus used a theoretical example in his dialogue, as John noted in 21:23.