Consul traditionally required client agents to be deployed on every host in the data center, and the services running on those hosts needed to be registered against the local client agent. Each client agent, or node, had a ID associated with it. This ID could either be manually specified by the operator, or automatically generated by the Consul agent on initial startup.
The Node field in the /catalog/register allows you to provide the ID of a node at which to associate the service. Typically node IDs are UUIDs. They can be generated using a tool on your computer such as uuidgen.
The /ui/<dc>/nodes page displays a list of nodes present in your Consul datacenter.
If you are using Consul 1.14+ on Kubernetes, Consul client agents are no longer required. You will not see any nodes displayed on this page, with the exception of the server agents.