What does lambda with 2 arrows mean in Java 8?

I have read several Java 8 tutorials before.

Right now I encountered following topic: Does java support Currying?

Here, I see following code:

IntFunction<IntUnaryOperator> curriedAdd = a -> b -> a + b;
System.out.println(curriedAdd.apply(1).applyAsInt(12));

I understand that this example sum 2 elements but I cannot understand the construction:

a -> b -> a + b;

According to the left part of expression, this row should implement following function:

R apply(int value); 

Before this, I only met lambdas only with one arrow.

If you express this as non-shorthand lambda syntax or pre-lambda Java anonymous class syntax it is clearer what is happening...

The original question. Why are two arrows? Simple, there are two functions being defined... The first function is a function-defining-function, the second is the result of that function, which also happens to be function. Each requires an -> operator to define it.

Non-shorthand

IntFunction<IntUnaryOperator> curriedAdd = (a) -> {
    return (b) -> {
        return a + b;
    };
};

Pre-Lambda before Java 8

IntFunction<IntUnaryOperator> curriedAdd = new IntFunction<IntUnaryOperator>() {
    @Override
    public IntUnaryOperator apply(final int value) {
        IntUnaryOperator op = new IntUnaryOperator() {
            @Override
            public int applyAsInt(int operand) {
                return operand + value;
            }
        };
        return op;
    }
};