Well it gets the HTML code of a particular URL :)
And it does the same thing as the FluentSharp BCL's .GET() extension method.
Here is a script that shows both in action:
This script (using the O2 Platform’s C# REPL) will create two source code viewers (side by side) and will show the content’s of both .html() and .GET() inside it, the return value confirms that they are the same:
Interestingly, google.co.uk’s response is unique (note the return value):