Seed nodes are nodes that are listed in the seednodes.fref file. These nodes must have the "be a seednode" config option enabled.
Seed nodes are used for opennet bootstrapping. An opennet node will, on startup, connect to seed nodes in its seednodes.fref file. It will use these connections solely to gather regular opennet connections through announcements, not for routing requests.
At present (build 1232) the seednodes.fref is not updated automatically, though it should be. Updates can be downloaded manually from http://downloads.freenetproject.org/alpha/opennet/