"loop:" in Java code. What is this, and why does it compile?

This code just made me stare at my screen for a few minutes:

loop:
for (;;) {
    // ...
}

(line 137 here)

I have never seen this before, and I had no idea Java has a "loop" keyword (NetBeans doesn't even color it like a keyword), and it does compile fine with JDK 6.

What is the explanation?