Rare is a catch-all label indicating that a bottle might be hard to find. A spirit is marked Rare when it is:
Limited edition,
Out of production, or
Limited to a specific geographic region
For example, an ultra-limited-edition single malt scotch and a craft gin from Japan with limited distribution would both qualify as Rare.
Keep in mind: Rare is not an indicator of quality, and it shouldn't be read as a suggestion to buy. It's simply a heads-up that tracking down the bottle may take some effort.