Nodes that generally have a high uptime and have had many connections to many other nodes. They are used by nodes that are first joining the network to get a list of other nodes to connect to. The DNS Seeders give out lists of seed nodes and there are also some hard coded fallback seed nodes in Bitcoin Core to be used if the DNS Seeders are unavailable.
Example Sentence :
I have launched the QT wallet and The seed node has provided a list of peers.
It is important to note that nodes do not connect to seed nodes as a normal node. Rather they connect, request IP addresses of other nodes (using the getaddr message) and then disconnect. Nodes then connect to the nodes that were given in the addr message (the response to getaddr).