This is a proposal to add a Bridge Hub (BH) parachain to Polkadot. All technical details can be found in the Bridge Hub Genesis release. The runtime is taken from the v9382 parachains release.
The BH will be the gateway between Polkadot and other networks, like Kusama and Ethereum. For now, the BH only contains some basic logic, and none of the bridge-specific pallets. Those will get added in future runtime upgrades; the goal of this proposal is to get approval from Polkadot stakeholders and launch a "shell" such that bridge teams can focus on releasing their bridge logic, not launching a parachain.
The BH will primarily act as a message routing system within the network and between others. For example, parachains can configure their XCMP routing to interpret parents: 2 (indicating, "go to another consensus system") as something to send to the BH to handle. Likewise, the BH can handle incoming messages and route them to the correct parachain within Polkadot.
Once operational, a Polkadot<>Kusama bridge will allow more interaction between the networks. For example, from the Polkadot Collectives parachains, the Technical Fellowship can serve both Polkadot and Kusama; or the Kusama network (as a unit body) could join the Polkadot Alliance.