attr_accessible如何理解

如题，看书看到的，但是并没有相应的解释

------------------------------------ ActiveRecord::Base::attr_accessible
  ActiveRecord::Base::attr_accessible(*attributes)
------------------------------------------------------------------------
  If this macro is used, only those attributes named in it will be
  accessible for mass-assignment, such as +new(attributes)+ and
  +attributes=(attributes)+. This is the more conservative choice for
  mass-assignment protection. If you'd rather start from an all-open
  default and restrict attributes as needed, have a look at
  attr_protected.