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It basically means that when you write your code, you don't need to worry about there being the same name somewhere else. All your resources - nodes, params, topics and services will be in the local scope.

And then you can write a top level node to include all those nodes without any problem.

Edit: (based on comment to answer)

Global: ROS has a global namespace - "/". If you define any of your graph resource names with a leading /, they will be global names. You can have a bunch of hierarchical namespaces but as long as it starts with a "/", it is globally resolved.

Example: "/global/name/with/base"

Base: That brings us to the second type - base names. Refer to the example above, every global name in ROS has a base which in this case is "base". You can look at it as the last name in the hierarchy and it doesn't have the "/"

Relative: ROS also has something known as a default namespace. The default namespace can be specified but if not specified it is the global namespace - "/". Relative names DO NOT have the leading "/" and are resolved WRT this default namespace.

Example: If the default namespace were "/global/name", then "with/base" would be the relative name. Notice that there is no leading "/" to "with/base"

Private: These names too are relative names, just that they use their nodes name as their default namespace. And they have to be specified with a leading "~".

Example: If "/global/name/with" was the name of the node, then the pvt. name "~base" would be the private name.