What does it mean when the hash tag/pound sign is placed before and/or after a word?

[StarSwinger]

I see this a lot and am confused. For example #cookies or #oaktree# Also why is it called "hash tag" and not pound sign like it was before?

[Stray]

I think it started with Twitter as a common way to look up posts that have similar concepts. For instance, if you were looking for posts about #cookies, it would all be pulled up because the user was thoughtful enough to label it with a hash tag.