A permissioned network is one where only authorized nodes are allowed to perform specific network activities.
The diagram shows you a network with 4 nodes: Alice and Bob are the nodes predefined in the genesis specification for genesis state which basically means the 2 nodes that will start the network.
In a permissionless blockchain anyone can join the network by running the node software, but in the case you want to build you have to add a new node as a well-known node. In the diagram Charlie and Dave has been added lately into the network.
To add these nodes in Substrate is recommended to use the node-authorization pallet.
The node-authorization pallet is a prebuilt FRAME pallet that enables you to manage a configurable set of nodes for a network.
For more practical knowledge I suggest you to go and build that network following these tutorials: Add trusted nodes and Authorize specific nodes. After this you will understand better the diagram, how to run a network and how to use the node-authorization pallet to add nodes into your network.