Hashtags are used on Twitter and other social media. It labels the subject of a tweet/post to make it easier to find in searches. A Twitter search for hashtags, for example, will bring up tweets containing #hashtag.
The # symbol was originally used to represent weight in pounds (it is still known as the pound sign by older users) It then appeared as the octothorpe on the early touch phones designed by engineers at Bell Labs in the 1960s - becoming steadily more familiar with the advent of automated switchboards, computer keyboards, mobile phones and many other joys (!) of modern life