Adobe Typekit Blog reports: [edited]
Jointly developed by Apple, Google, Microsoft, and Adobe, a variable font is “a single font file that behaves like multiple fonts”. A single font file gaining an infinite flexibility of weight, width, and other attributes without also gaining file size.
The OpenType font file specification now includes a new technology: OpenType Font Variations, which allows type designers to interpolate a font’s entire glyph set or individual glyphs along up to 64,000 axes of variation (weight, width, etc.), and define specific positions in the design space as named instances (“Bold”, “Condensed”, etc.).
For fonts to actually show up anywhere, a rendering engine has to make typesetting and rasterisation calculations. Rendering engines are complex, and will need to be developed before Variable Fonts become viable, along with browsers and design software to support the rendering engine.