Maybe I have been living under a rock, but I have only learned about this yesterday and thought I would share it with you guys.
Well, an images says more than … and an animated one is even better:
So it’s a new type of font file that includes all the font weights and variations in one file. Actually only the ‘regular’ and the extreme cases of a variation are there, and the ones in between are generated. e.g. font weight, that would be regular, thin and bold included and the rest generated:
Besides that, letter width, height, slant/italic can be variations, but also custom properties.
Variable Google Fonts