Its a Magi only building, you can see it in the guide if you click the Magi tab at the top.
The effect it has on game is that your merchants regenerate faster.
The -0 on prerequisites is a bug where the guide is not picking up properly that you can only have 1 per player.
Same for:
- 'Holy Grove'
- 'Cloister'
- 'Glass Maker'
- 'Vintner''s Shop'
Is on list of things to do to improve guide along with:
- stating unit types (infantry, mounted etc)
- terrain prerequisites
- resource field prerequisites
- Better indication of exclusive buildings