From the Plex blog:
Think of it as a way to have events in Plex affect the outside world. We’ve made it so that any media playback event or media rating event can trigger a webhook. If you’re into home automation, you know how cool and powerful this can be: dim the lights when you start a movie, or send a tweet after you finish an episode of Scandal. Working with other home automation tools, you can even have Plex turn on your A/V receiver or close your smart drapes automatically as you sit down for an evening of Plexing.
The home of the future is eking closer every day. I'd really like Alexa to gain access to my Plex library for voice controlled music streaming but this feels like a big step forward for Plex users. Here's where Amazon is smart. Alexa isn't just the command receiver, it's the integration hub. When services (like Plex) integrate through Alex to control each-other, Amazon becomes indispensable and it creates a barrier to moving away. I'm not as bullish about Alexa as many tech-writers seem to be. It's not more "intelligent" in my daily use of the Echo. It's just more convenient.