Webkit is the rendering engine that runs Apple Safari and Google Chrome. Analogously, Trident powers IE and Gecko powers Firefox. Webkit, however, is by far the most advanced of the three (with Trident being a distant third).
-webkit-transition: color 1s ease-in-out;
Enhancements like this are progressive-style enhancements – they aren’t supported everywhere, but they likely will be in the web of the future (minus the ‘-webkit’ identifier), and they don’t take much away if they aren’t present.