Ruby: what does :: prefix do?

The :: is the scope resolution operator. What it does is determines what scope a module can be found under. For example:

module Music
  module Record
    # perhaps a copy of Abbey Road by The Beatles?
  end

  module EightTrack
    # like Gloria Gaynor, they will survive!
  end
end

module Record
  # for adding an item to the database
end

To access Music::Record from outside of Music you would use Music::Record.

To reference Music::Record from Music::EightTrack you could simply use Record because it's defined in the same scope (that of Music).

However, to access the Record module responsible for interfacing with your database from Music::EightTrack you can't just use Record because Ruby thinks you want Music::Record. That's when you would use the scope resolution operator as a prefix, specifying the global/main scope: ::Record.

module A
  def self.method; "Outer"; end
end
module B
  module A
    def self.method; "Inner"; end
  end
  A.method   # => "Inner"
  ::A.method # => "Outer"
end

On the specific case of Artifice, at line 41 of the file that you've shown is defined a inner Net module. To keep acess to the outer Net module, it uses ::Net.

A :: operator refers to the global scope instead of local.