what is meant by this: def foo=(foo) ... end?

I saw in episode #75 that creating a method in the Product model will
update attributes related to a different model (Picture):

def picture_attributes=(picture_attributes)

I'm unclear as to the first line:

def picture_attributes=(picture_attributes)
def picture_attributes (picture_attributes)

def picture_attributes=

defines the setter, whereas

def picture_attributes

defines the getter

It is unusual for a getter to take a parameter because getters are not supposed to have side-effects, just return a value. More rubyish names for these functions are "accessors" so declaring:

attr_reader :picture_attributes

automagically defines a getter (http://ruby-doc.org/core/classes/Module.html#M001702).

attr_writer :picture_attributes

automagically defines a getter that takes one value argument (http://ruby-doc.org/core/classes/Module.html#M001703).

attr_accessor :picture_attributes

automagically defines both (http://ruby-doc.org/core/classes/Module.html#M001704).