This allows for Java edition servers to have public IPs exposed in many geo-regions, and to send a packet to the client to hop to another server. Normally proxies like Bungeecord or Velocity have to be used to link servers together, but now there is another method that does not require everything to be routed through one IP address, or use anycast setups.
From Mojang's changelog:
### Transfer Packets
- Custom servers can now request that clients connect to another server with a new packet
- When a client is transferred it will connect to the target server with a new transfer intent (id 3)
- By default servers will not accept incoming transfers and will disconnect the client
- This can be changed by setting the accepts-transfers property to true in the server.properties file
- Resource packs are maintained across transfers
- In the case of a transfer custom servers can skip authentication with a new flag
### Cookie Packets
- Cookie packets allow custom servers to request and store data on a client
- - Each cookie may be up to 5 KiB in size
- - Cookies may be requested during login, configuration and play phases --- but only stored during the configuration and play phases
- Cookies are persisted across server transfers but are not persisted when the player disconnects
- - This allows servers to pass along information such as authentication or custom game data to the new server
From Mojang's changelog:
### Transfer Packets
- Custom servers can now request that clients connect to another server with a new packet
- When a client is transferred it will connect to the target server with a new transfer intent (id 3)
- By default servers will not accept incoming transfers and will disconnect the client
- This can be changed by setting the accepts-transfers property to true in the server.properties file
- Resource packs are maintained across transfers
- In the case of a transfer custom servers can skip authentication with a new flag
### Cookie Packets
- Cookie packets allow custom servers to request and store data on a client
- - Each cookie may be up to 5 KiB in size
- - Cookies may be requested during login, configuration and play phases --- but only stored during the configuration and play phases
- Cookies are persisted across server transfers but are not persisted when the player disconnects
- - This allows servers to pass along information such as authentication or custom game data to the new server