What does @link mean in java/Android documentation?

It's an annotation used so when you generate the Javadoc, you'll have a link for the specified item. In this case, the KeyEvent class, with the anchor of KEYCODE_ENTER.

I have not formally learned so perhaps someone can expand upon what I say.

It is shorthand that tells the java docs to insert a link to the desired place when they are being viewed. For instance when you view the javadocs for whatever method has that inside your IDE you'll be shown a link that will take you to the KeyEvent.KEYCODE_ENTER javadoc upon being clicked.

With @link you can link to the Javadoc of a different class or class-member. See also the Javadoc documentation for @link

When the Javadoc is generated, a link to the right page + anchor is inserted. This allows for cross-referencing other parts of the Javadoc.