No knight: 
Node is down or invalid.
Knight standing: 
Node is up.
Knight on horse, with check mark: 
Node is up, metrics have been selected for monitoring.
The nodes area, on the left side of the main window, displays icons for nodes you can monitor. By default, the local node is displayed and belongs to the group World.
To manage the nodes, clusters, and groups appearing in the nodes area, use the toolbar or go to the main window's Tasks menu and choose Node Management.
A node is a computer system that is uniquely addressable on a network. A node can have more than one CPU. Single horses represent individual nodes in various states. Note that a node icon may take a few moments to reflect the state of the node after the node is newly added or comes up. A node icon changes to reflect one of the following three node states:
No knight: Node is down or invalid.
Knight standing: Node is up.
Knight on horse, with check mark: Node is up, metrics have been selected for monitoring.
A cluster is a collection of nodes that appear to be a single-server system. Clusters offer
application availability and scalability greater than is possible with
a single system. A set of three horses 
represents a cluster.
A cluster icon changes to reflect one of the following three cluster states:
No knights: All nodes in the cluster are down or invalid.
Knights standing: At least one node in the cluster is up.
Knights on horses: Nodes are up, and, if there's a check mark, metrics have been selected for monitoring.
A check mark indicates that metrics have been selected for monitoring. In addition, when a cluster is selected, the background color of the cluster icon will change.
A group is a collection of nodes and/or clusters that are frequently managed together.
Castles 
represent these collections.
If the group icon shows a check mark, metrics have been selected for monitoring for every cluster and node in the group. In addition, when a group is selected, the background color of the group icon will be changed.
A shield appears next to each castle (group) and set of three horses (cluster).
A shield displaying a minus sign (-) 
shows that all nodes
in the group or cluster are exposed.
A shield showing the plus sign (+) 
shows that all nodes are hidden.
Clicking on that shield would expose all the nodes and clusters inside.