Hashtags are a form of metadata used to cross-reference content by adding the hash symbol (#) as a prefix to specific segments of text. A search for a specific hashtag will return all the public posts that included it. Widespread use of a hashtag on social media can be indicative that the topic it refers to is trending, though this also means that some users will attempt to draw attention to a topic by spamming its hashtag to make it appear more popular than it actually is.
Sometimes hashtags are used to provide additional context around a given message, often for humorous effect. For example, a post with the message “Happy Monday!” might include #scarcasm if the poster is being facetious. Hashtags have also been used to express political views or make social statements, such as #MeToo or #BLM.