As a first-time user,I didn't, and just wrote out consequetive keywords, then clicked "POST".
When I then went back to edit my post, all my tags had disappeared. I thought it was a bug.
Only found out how to add tags by trial and error.
Changing the help sentence above the input box would have helped, from...
"Tag your post with keywords below; numbers shown in parentheses represents popularity."
...adding something like...
"Hit Enter after each keyword, to add it to the post."