I'm trying to figure out what hashtags actually are.

Seen as keyword metadata, they are misplaced because they're not stored separately. Also, when words in a sentence are hashtagged, hashtags serve as data and metadata simultaneously.

Seen as a markup language, hashtags make more sense, except for the fact that markup syntax itself is almost always invisible once a document is rendered. Hashtags, on the other hand, are visible on purpose and figure prominently in pop culture.