#, as you are describing it, is called "hashtag", or simply "tagging".
you see, when you post a video or photo, they do not contain words, just pictures.
the # sign is used to "tag" them with key words used or applying to the pictures or video.
so a scene with Santa giving kids presents might be tagged #Christmas2018 or #Santa or #presents, etc. they may also tag the names of people in the video or photos.
alternately, it may be used on services like Facebook or Twitter when the post is ABOUT something, but does NOT use the hashtag words IN the post. so a post about Trump might be tagged #Trump or #DonaldT or #Drumpf (by his haters).
then, people who post similar statements might use the same tag, and you can find like-minded people easily.