What's $ operator supposed to mean for a string?

So, I've just had the following conversation with a user in the comments section.

Me: 

Year year = new Year{ State = States.Happy };

Them: 

eventStream.ReceiveJoke += joke =>  
    Console.WriteLine($"Pretty nice joke: {joke}, Thanks!!!");

And, nerdy as I am, I wonder what he meant by the dollar sign but I think it's too embarrassing to ask him.

Jon Skeet
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quotationmark

It's an interpolated string literal, introduced in C# 6.

It's broadly equivalent to:

eventStream.ReceiveJoke += joke =>  
    Console.WriteLine(string.Format("Pretty nice joke: {0}, Thanks!!!", joke));

The compiler looks for braces within any string literal introduced with $, and applies string formatting to it. You can use (mostly) arbitrary expressions, rather than just variables, e.g.

Console.WriteLine($"{year.State} {2000 + 16}"); // Happy 2016

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