This certainly isn't short, but I don't know how to make it shorter, as I don't know what you don't know, and what you want to know.
You need a DRM "capable" video component to be allowed to play most internet video content that isn't directly user generated, or news.
The consequence is that the giants holding the rights to these components are effectively holding the keys to a decent chunk of the internet. For a browser specifically made for synced playback of movies, they essentially hold the keys to all of it. The big movie studios simply won't allow either streaming or download without you using one of these solutions on both ends of the stream.
The author applied for some form of license of the purportedly free and open Widevine DRM component which is used/approved by almost all big streaming sites. The request was rejected citing a somewhat odd reason: That the project was open source.
This is odd because https://github.com/castlabs/electron-releases is open source, apparently "blessed" by Google/Widevine, and available to use on the only condition you get a license from Widevine. Which should then be impossible?
Which leads us to several somewhat plausible conclusions. Either the author requested the wrong licence/needs a feature not available in previously mentioned projects, someone at Widevine made a mistake, or there is a lot of smoke and mirrors going on to either make it seem Widevine is accessible to anyone when it truly isn't.