In the world of social media, a hashtag symbol (#) creates a searchable topic of discussion about a person, event or thing. Hashtags are particularly used on Twitter, where people use it to discuss topics, ranging from current affairs to entertainment.
When people use a hashtag followed immediately by a word or phrase, it allows the message to appear when other users enter searches around that topic. For instance, during the Super Bowl, using #SuperBowlCommercials allows a person's message to appear in inquiries related to Super Bowl commercial Tweets. Highly used hashtags often show up as "Trending Topics," making them more visible to users.