Hashtags have become an essential part of social media. They were originally used as a way to tag or mark topics in a post on social media. For example I could tweet about a library event without a hashtag which would be perfectly acceptable. But if I added the hashtags #Oamaru #Library and #NZ it could increase the amount of people who would see the tweet, because anyone who searches for those hashtags will see the post. They can also be used as a professional development tool, for example, following the hashtag from a conference to see all the tweets.
Below are some resources that go into more detail about how to use the hashtag like a pro: