Hashtags are prevalent on many social media platforms these days. With regards to Google's own services, Google+ had hashtag support before it was killed off, and YouTube has had them for nearly three years at this point. Discover is the latest Google service to experiment with hashtags, though it does seem a little unnecessary.
The implementation in Discover is somewhat similar to how YouTube shows them, with a couple of hashtags showing up near the bottom of each card. In my opinion, these hashtags are kind of useless; the title does a much better job of telling me whether I actually want to read the article or click on the video.
That said, if you do want to see more content about said hashtag, you can tap on it to see more relevant articles and videos. In the screenshots above, tapping on the #FlatSixEngine hashtag shows some more content that Google thinks you'll like, but in reality, I doubt people are looking for things to read or watch like this.
For now, these hashtags seem to be part of a fairly limited A/B test. I'm personally hoping that they don't become official, as they just add clutter.