A hashtag is simply a keyword phrase used on social networks and microblogging services with a pound sign(#) in front of phrase without spaces.
It helps in categorizing messages. Provides a way for the public to express their sentiment. It is a very powerful marketing tool as it goes beyond just your followers. It makes possible for others to easily find and follow a specific theme or content.
Almost all social media platforms support hashtags. As these become followable links, Use of proper hashtags definitely boosts your audience reach.