Tags.pub is a global hashtag server for the Fediverse
If you post a note, image, video, audio, or article publicly on the Fediverse and include a hashtag, the content will be boosted by the hashtag account at tags.pub. You should get a notification of the boost in your Mastodon or other Fediverse server. tags.pub will never boost any followers-only posts or direct messages; in fact, your server will not send them.
If you edit the hashtags of your content, the tags.pub accounts for the tags you add will boost the content, and the accounts for the tags you remove will undo the boost. You'll get a notification of it either way.
If you delete your content, all the tags.pub accounts that boosted it will un-boost it.
If you make a post with a hashtag, and it isn't boosted by tags.pub, it probably means that your server is not connected to tags.pub. Check with your server administrator.