I'm assuming flar_ is your prefix?
In the tag_user table, the columns marked_as_read_at and is_hidden might not actually be implemented. I believe the first is supposed to handle per-tag "mark all as read" and the second is supposed to be a way for the user to hide all messages from a tag from the homepage. But I don't see any actual code for those features so it's probably some of those planned features that were never finished since the launch of Flarum.
The table itself acts as a pivot table for extensions to add columns to whenever they need a user-tag pivot. For example the "follow tags" extension adds a column for the subscription state.
