Metadata is the pernicious transaction data involving the "To", "From" and subject fields of email that the NSA finds so valuable for tracking communications and drawing connections between people. Generally, even when email is encrypted, metadata is not. Dark Mail ambitiously aims to revamp existing email structures to hide this data while still making the system universally compatible with existing email clients.
The project has made for an interesting pairing between Texas technologist Ladar Levison and convicted hacker Stephen Watt, whom he's hired to help develop the code. Both have had previous battles with the government in very different ways.