In short a variable font contains all of the font permutations within a single font file. These variations are exposed through an axis of variation which can be set in CSS. The font creature can expose any axis to be manipulated, including custom defined ones.

some information and examples:

• https://developers.google.com/web/fundamentals/design-and-ux/typography/variable-fonts/

• https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Fonts/Variable_Fonts_Guide

• https://developer.microsoft.com/en-us/microsoft-edge/testdrive/demos/variable-fonts/

• https://v-fonts.com

• https://www.axis-praxis.org/specimens/_DEFAULT_