In this code, dd (without quotes) is a variable that holds a value of 3.2. But "dd" (with quotes) is a String literal that has nothing to do with the variable or its value. So "dd" has no meaning as a float.
"3.2" (with quotes) is also a String literal, but it is in a format that can be parsed to a float.
"We're kind of on the level of crossword puzzle writers... And no one ever goes to them and gives them an award." ~Joe Strummer sscce.org