What is the ‘open’ keyword in Swift?

open is a new access level in Swift 3, introduced with the implementation
of

• SE-0117 Allow distinguishing between public access and public overridability

It is available with the Swift 3 snapshot from August 7, 2016,
and with Xcode 8 beta 6.

In short:

• An open class is accessible and subclassable outside of the
  defining module. An open class member is accessible and
  overridable outside of the defining module.
• A public class is accessible but not subclassable outside of the
  defining module. A public class member is accessible but
  not overridable outside of the defining module.

So open is what public used to be in previous
Swift releases and the access of public has been restricted.
Or, as Chris Lattner puts it in
SE-0177: Allow distinguishing between public access and public overridability:

“open” is now simply “more public than public”, providing a very simple and clean model.

In your example, open var hashValue is a property which is accessible and can be overridden in NSObject subclasses.

For more examples and details, have a look at SE-0117.