The feature is for the Android app, and enables some handy in-car features. It enlarges the forward and back buttons, so they're easier to see at a glance. It also announces songs, so you can know what's playing without having to take your eyes off the road, and will ultimately enable voice controls (these aren't active yet).
A car emblem appears in the bottom corner of the screen, which you press to enable Driving Mode.
According to the user who posted screengrabs, the feature asks for an '@spotify.com' email address, suggesting it was only meant to be tested by the firm's employees at this stage. But somehow it's made it out to wider users.
As to when we'll actually see this feature enabled, we'll have to wait and see. But anything that makes it safer to listen to music while driving gets the thumbs up from us.