Hole-punching is a technique used by two different computers with routers to facilitate data transmission. Without it, data can't flow through the routers as they do not allow incoming packets unless an outgoing packet is sent. Hole-punching overcomes this by allowing information to go both in and out. With hole-punching, direct connections are typically established instead of having to use an intermediary service which would result in meta data leaking. When each node wants connect to the other, they will send messages through public or relay servers that explain their desire for a connection. By using this method, both parties can choose who they want to talk directly and from there the holepunching allows the rest of the data to be sent and received. For general understanding what is NAT you can also check [this post](https://pl-launchpad.peeranha.io/experts/1437/how-does-nat-traversal-work%3F)