What does question mark and dot operator ?. mean in C# 6.0?

With C# 6.0 in the VS2015 preview we have a new operator, ?., which can be used like this:

public class A {
   string PropertyOfA { get; set; }
}

...

var a = new A();
var foo = "bar";
if(a?.PropertyOfA != foo) {
   //somecode
}

What exactly does it do?