This ‘#’ is a hashtag. You add it to the beginning of words to tag the word, like so: #hashtags. Putting any spaces or punctuation will break your hashtag, so don’t put any in yours. Capitals are okay, and a good idea to help differentiate words in longer hashtags.
What do hashtags do?
Hashtags are used to make things more searchable on social media. Adding a hashtag to your post will group it with other posts that contain the same hashtag. So when somebody searches for that hashtag or keyword, your post will show up along with all the other posts with that hashtag.
Hashtags make it easy for you to connect yourself to relevant discussions, join trending topics, and they make your posts more searchable. They are especially important on Twitter and Instagram, where they are most popular. But they are also helpful on Facebook and other channels.