RAID 5 is block-level striping with distributed parity while RAID 6 is block-level striping with double distributed parity. RAID 5/6 support has been talked about for Btrfs for a long-time while this weekend the experimental patches were finally pushed by Chris Mason.
Still to be worked out is a few bugs plus support for scrub repairing bad blocks, discard support, progs support for parity rebuild, and planned support for n-way mirroring.
Many more details on the implementation and other information regarding Btrfs RAID 5/6 can be found from the mailing list announcement. While currently an experimental feature, the support will likely end up in a mainline Linux kernel release in one or two cycles ahead.