Sign ‘#’ 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. 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.
How to use them on Facebook?
Hashtags turn topics and phrases into clickable links in your posts on your personal Timeline or Page. This helps people find posts about topics they’re interested in. This is really simple – when you click a hashtag, you’ll see a feed of posts that include that hashtag. Using them may help your Page to be more visible for people, who are actually really interested in the topic, that you are writing about. But remember – they are not as useful as hashtags in Tweets. Even if someone type particular hashtag in searchbox, firstly he would find those posts, which are written by their friends or Pages they like, using this hashtag.