Horizon uses Whisper for secure private peer-to-peer communications. Whisper was developed as part of the Ethereum project. Whisper is conceptually similar to Telehash enabling secure identity-based messaging. Whisper is also designed to efficiently support broadcast messaging.
Since everything on the blockchain is publicly readable, Horizon required a private messaging service to enable the secure exchange of sensitive data between participants (e.g., to pass personally identifiable information, such as GPS coordinates, IP addresses, etc.). Whisper is typically used to exchange additional private details once participants have discovered each other, proposed and accepted a contract.
Note that Whisper should not be used to exchange high volumes of data. Whisper should be considered a control channel, and some other transport should be used on your data channels.
If you wish to get more information about Whisper technology, you may find the links below useful.