A string is anything in python surrounded in single or double (or triple) quotes. The “argument” is what gets passed to the function. In this case the function is open_form and the argument is 'Form2' (notice the single quotes).
Compare what I have written above to what is in your code. You will see that you are calling Form2 as a variable (which is not defined, hence the error). I have enclosed Form2 in single quotes, making it a string. That is what the function open_form takes inside its parentheses.