Tell me what it means to add two # s to QQ space, such as "# what what what #"

Talk about adding two # s and a keyword in the middle. This is the usage of the theme tag.

The earliest label was #. Later, foreign Weibo and social networking sites added hyperlinks to the labels. Click to open the topic aggregation page with the same label. In Weibo and social networking sites in China, only users are used to using double # to mark topics.