What is line weight? Isn't it the size of the text? Can't I just make the font size a big number?
Line weight is the thickness of a line. It is not a CSS property.
Line-height is a CSS property, and indicates the height of an inline box. Yes, increasing the font-size will also increase the size of this box, but the line-height property will allow you to change the size of the box without changing the font-size.