When Nagios says a node is UNREACHABLE, a node is unreachable if Nagios is not able to FIND a path to the node. 
Now you can mention the difference.

The node itself may be up but because Nagios is unable to connect to it, it has to mark this as unreachable. To achieve this, Nagios use parent-child relationship between COMPONENTS.

Finally for better understanding explain it with an EXAMPLE.

• A router may be defined as a parent for a server.
• Now Nagios checks for server and marks it as down.
• It then checks the parent (in our example, the router)
• If parent is also down, then server is MARKED as unreachable.
• If Parent is up, the server is marked as really down.

When Nagios says a node is unreachable, a node is unreachable if Nagios is not able to find a path to the node. 
Now you can mention the difference.

The node itself may be up but because Nagios is unable to connect to it, it has to mark this as unreachable. To achieve this, Nagios use parent-child relationship between components.

Finally for better understanding explain it with an example.