Chrome whitelisted some of the most popular NPAPI plug-ins like Silverlight or Google Talk. As their usage continued to decline, the whitelist will be removed in January and users will have to manually enable the plug-ins.
As you can see from the table below, the only plug-in used by more than 10% of the Chrome users is Silverlight and it's followed by Google Talk, which is still used by 7% of the Chrome users. Java usage declined from 8.9% to 3.7%, Facebook's plug-in usage declined from 6% to 3%, while Unity is only used by 1.9% of the Chrome users, down from 9.1% in September 2013.
In April 2015 NPAPI support will be disabled and Google will unpublish from the Chrome Web Store the extensions that require NPAPI plugins. Power users and business users will still be able to enable NPAPI using Chrome flags or Enterprise Policy, but only until September 2015, when NPAPI support will be completely removed. There's a deprecation guide for developers which offers a few alternatives to NPAPI, including HTML5, WebRTC, Chrome APIs for apps and extensions and Native Client.
NPAPI is a legacy technology that enabled a lot of powerful features, back when browsers couldn't play videos, handle video calls or run games. You had to install QuickTime or RealPlayer to play videos, install plug-ins for Google Talk or other video calling apps, install Java or Flash to play games. Now browsers are a lot more powerful and the features that are still not supported by Chrome can be enabled by more secure NPAPI alternatives like PPAPI and Native Client, which are unfortunately still only available in Chrome.