divTwoWidth is a variable, that has value set earlier. When setting the width of this element, you want to set it to the value recorded earlier in the divTwoWidth variable. To do that, you need to access the variable itself. If you put the variable in quotes, you get a string with the value 'divTwoWidth', which is something else entirely.
divTwo.css('width', divTwoWidth + 'px');
That will set the width of
divTwo, because the value of
divTwoWidth is 200.
Had you used (incorrect) the string variant, you'd have
divTwo.css('width', 'divTwoWidth + px');
set the width to 'divTwoWidth', but 'divTwoWidth' is not a number, so the expression is wrong (and will probably set the width to 0 since the string 'divTwoWidth' is 0 when cast to an integer).
Does that make sense?