In short, the double colon in Java 8 is method reference operator, which can be used to get the reference of an instance method or a static method, then the result of the operator, aka the method reference can be used in the funtional style programming supported in Java 8.
here gives a very clear explaination.
Added by 2018-05-28: Note that there is some limitation when using method reference with methods in a inner class, please read this.