OpenType variable fonts are an extension to the OpenType specification, allowing a single font file to store a continuous range of design variants. What does that mean? Simply put, instead of eight separate files to cover the range of weights in Plex Sans, you get one file containing all weights. This decreases the combined size of font data and thus makes for faster page load times (equals money).
Because the font technology uses interpolation and extrapolation mechanisms, it offers the user with built-in variability. You not only get the static weights, you also what lives inbetween. With mechanisms exposed to the user, they can be built into operating systems and browsers or other applications, allowing for responsive web and app design.
Although Adobe, Apple, Google, and Microsoft already announced the technology in September 2016, it took a while before it was adapted to major platforms. Around late 2017 early 2018, variable fonts could be used in software and browsers on OS X and Windows.
IBM Plex Sans Var was released April 2019. As part of the Bold Monday team, I was largely involved in the development of the variable font. This included providing the font with TrueType hints (instructions for the rasterizer) in Microsoft VTT as well as building a demo site (for internal use) to test and showcase the font. The demo was built to work cross platform in Chrome, Edge, Firefox and Safari.
Special thanks: Mick Duggan and Max Peeperkorn.
IBM Plex has been released as open source under a SIL OFL 1.1 license and can be downloaded from the IBM GitHub page and is also available through Google Fonts. The project is big — for a closer look at the whole project and concept have a look at the IBM Plex website.