As @Jeff-UK already commented, @johan talked a lot about Packet Broker during The Things Conference in February 2020. Here in the opening keynote, and later during this presentation. You should definitely watch those videos.
Packet Broker enables bi-directional peering between networks. You can configure routing policies that tell Packet Broker what kind of uplink traffic to forward to other networks (or a specific network) and what kind of downlink traffic to accept from other networks (or a specific network). An example policy would be to forward all uplink traffic (because you already received that anyway), and allow MAC downlinks (so that the Network Server can perform ADR).
We have implemented the “forwarder” role in our v2 stack, which will help with a smoother transition from v2 to v3. Since the public community network still runs on v2, you’ll only observe this “forwarding” behavior there. The Things Stack (v3) supports both the “forwarder” and “home network” roles. This is how v3 clusters can exchange traffic between each other. When we start rollout of the v3 public community network you’ll also start seeing incoming Packet Broker traffic there.