It's the scope resolution operator.
An example from Wikipedia:
module Example
Version = 1.0
class << self # We are accessing the module's singleton class
def hello(who = "world")
"Hello #{who}"
end
end
end #/Example
Example::hello # => "Hello world"
Example.hello "hacker" # => "Hello hacker"
Example::Version # => 1.0
Example.Version # NoMethodError
# This illustrates the difference between the message (.) operator and the scope
# operator in Ruby (::).
# We can use both ::hello and .hello, because hello is a part of Example's scope
# and because Example responds to the message hello.
#
# We can't do the same with ::Version and .Version, because Version is within the
# scope of Example, but Example can't respond to the message Version, since there
# is no method to respond with.
