Variable fonts are an evolution of the OpenType font specification that enables many different variations of a typeface to be incorporated into a single file, rather than having a separate font file for every width, weight, or style.
The heart of the new variable fonts format is the concept of an axis of variation describing the allowable range of that particular aspect of the typeface design. So the ‘weight axis’ describes how light or how bold the letterforms can be; the ‘width axis’ describes how narrow or how wide they can be; the ‘italic axis’ describes if italic letterforms are present and can be turned on or off accordingly, etc. Note that an axis can be a range or a binary choice. Weight might range from 1–999, whereas italic might be 0 or 1 (off or on).