I was a bit confused by the double Function call too. The string input to the first Function is:
"return"ale\162t(\"\111\40lo\166e\40\171ou.\"\40)""
which when evaluated becomes: "alert("I love you." )"
He gets the other letters from names of types and other JS native things. I guess he couldn't get 'r', 'I', ' ', 'v' or 'y' and so had to make them using the octal escape codes.
So he's doing 0["constructor"] which gives the constructor of a number, Number. Then he's 0["constructor"]["constructor"] gives the constructor of Number, which is like a function, so he gets Function.
Then he basically does this ($.$ == Function): The inner function just returns the string "alert('I love you');", which then becomes the body for the outer function.I don't know why they didn't make it:
Function ("alert('I love you');")();
Maybe they needed more chars for the heart. That was a good little crash course in javascript Function constructors!