As the name explains “non-routable VLAN” is a virtual LAN that you cannot route. VLANs are by default Layer 2, restricting the broadcast domain. If you create a VLAN it will be Layer 2 until the time you assign an IP address to it, which makes it routable.
You are right in saying that it’s a VLAN without any IP address assigned to it. The reason for doing this is for security issues. All VLANs which you don’t want to route to should be non-routable.
VLAN hopping is one of the security issues that would make you want to restrict access between VLANs and use non-routable VLANs. Some of my customers have liked the idea of using non-routable VLANs for Internet access/guest users.
Hope this clarifies.