ESV - 18 Truly, I say to you, whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Whoever knows and belives in Jesus as Peter did,(Mathew 16: 16-17) will become the children of God (John 1: 12-13), an ambassador of the kingdom of heaven to the world.
I have a NAS Bible and, thinking about the other gospels as well, I understand that it means: retaining and forgiveness of sins.