Say my friend has a sister named Sue, and so do I. How can I tell them apart in a conversation? By using their last name. The last name is the namespace.
Modules define a namespace, a sandbox in which your methods and constants can play without having to worry about being stepped on by other methods and constants.
1 2 3
Dog::Species => Species is a Constant within the Dog class. Species could be a class it could be a module, but it is a child of Dog.