What is the difference between font-size and font-weight?

Answer 1

Weight is if font is more or less bold.
Size is height of font...

Answer 2

The font-weight property sets how thick or thin characters in text should be displayed.
syntax:
font-weight: normal|bold|bolder|lighter|number|initial|inherit;

The font-size property sets the size of the text.
Being able to manage the text size is important in web design. However, you should not use font size adjustments to make paragraphs look like headings, or headings look like paragraphs.