$ is short-hand for String.Format and is used with string interpolations, which is a new feature of C# 6. As used in your case, it does nothing, just as string.Format() would do nothing.

It is comes into its own when used to build strings with reference to other values. What previously had to be written as:

var anInt = 1;
var aBool = true;
var aString = "3";
var formated = string.Format("{0},{1},{2}", anInt, aBool, aString);

Now becomes:

var anInt = 1;
var aBool = true;
var aString = "3";
var formated = $"{anInt},{aBool},{aString}";

There's also an alternative - less well known - form of string interpolation using $@ (the order of the two symbols is important). It allows the features of a @"" string to be mixed with $"" to support string interpolations without the need for \\ throughout your string. So the following two lines:

var someDir = "a";
Console.WriteLine($@"c:\{someDir}\b\c");

will output:

c:\a\b\c

It creates an interpolated string.

From MSDN

Used to construct strings. An interpolated string expression looks like a template string that contains expressions. An interpolated string expression creates a string by replacing the contained expressions with the ToString represenations of the expressions’ results.

ex :

 var name = "Sam";
 var msg = $"hello, {name}";

 Console.WriteLine(msg); // hello, Sam

You can use expressions within the interpolated string

 var msg = $"hello, {name.ToLower()}";
 Console.WriteLine(msg); // hello, sam

The nice thing about it is that you don't need to worry about the order of parameters as you do with String.Format.

  var s = String.Format("{0},{1},{2}...{88}",p0,p1,..,p88);

Now if you want to remove some parameters you have to go and update all the counts, which is not the case anymore.

Note that the good old string.format is still relevant if you want to specify cultural info in your formatting.

Example Code

public class Person {
    public String firstName { get; set; }
    public String lastName { get; set; }
}

// Instantiate Person
var person = new Person { firstName = "Albert", lastName = "Einstein" };

// We can print fullname of the above person as follows
Console.WriteLine("Full-Name - " + person.firstName + " " + person.lastName);
Console.WriteLine("Full-Name - {0} {1}", person.firstName, person.lastName);
Console.WriteLine($"Full-Name - {person.firstName} {person.lastName}");

Output

Full-Name - Albert Einstein
Full-Name - Albert Einstein
Full-Name - Albert Einstein

It is Interpolated Strings. You can use an interpolated string anywhere you can use a string literal. When running your program would execute the code with the interpolated string literal, the code computes a new string literal by evaluating the interpolation expressions. This computation occurs each time the code with the interpolated string executes.

Following example produces a string value where all the string interpolation values have been computed. It is the final result and has type string. All occurrences of double curly braces (“{{“ and “}}”) are converted to a single curly brace.

string text = "World";
var message = $"Hello, {text}";

After executing above 2 lines, variable message contains "Hello, World".

Console.WriteLine(message); // Prints Hello, World

Reference - MSDN