Variable Fonts
Officially known as OpenType Font Variations.
More flexible
It removes the explicit distinctions between different weights and styles, which have existed since the early days of typesetting.
With variable fonts, the choice of exactly how heavy or slanted or wide (or any other parameter) type should be is placed into the hands of the user, rather than having those decided for us by the type designer.
Smaller file size
Traditionally, all possible weights and styles have been separated out into different font files, whereas variable fonts combine all of those variations into one. Because of this, overall file size is greatly reduced compared to loading multiple individual font files