A node is a connected CJDNS router.
To be precise, a node is a device with CJDNS installed, configued and connected to some peers.
All sorts of devices can be used as nodes.
For example, a device can be an actual router (with OpenWRT), possibly a wireless router. Or it can be a PC or server with a local CJDNS installation.
You can join too!
To connect to Hyperboria, you need to set up a node and connect to some trusted peers. You can find more information and tutorials on getting connected.