Question

which is a mean by '#' this​

Answer 1

Answer:

hashtag.............

Answer 2

a word or phrase preceded by a hash sign (#), used on social media websites and applications, especially Twitter, to identify messages on a specific topic.

"spammers often broadcast tweets with popular hashtags even if the tweet has nothing to do with them"

the hash sign (#).

"Hunt mistook an @ for a hashtag while tweeting derogatory thoughts about him"

Hope it will help :)