C # Operator ?? Do you know?

FromCodeIs there suddenly ?? Operator, maybe I am a little white. I checked msdn to find out that this operator is available. See the following example:

VaR Username = Session [ " User " ] ??   " Anonymous "

Obviously ,?? The operator is used:

If??The left operand is not null. This operator returns the left operand. Otherwise, the right operand is returned.

The above example should be frequently used for website development.