Q:

What does def <<(path) mean in rails/ruby

While looking through gem file for Grackle, found this method:
def <<(path)
  self.path << path
end

What does it mean and do? 

A:

It's a definition of << method, which could also be used in "operator" manner, like this:
a << path

It's commonly used syntax sugar in Ruby. In fact, if you do
2 + 4

you're really calling + method on 2 object with parameter of 4, like this:
2.+(4)

You can check on your own, that this last notation also works correctly. 

