From TheWarWiki

In every country there are tales of great beasts and daemonic lairs that have consumed many would-be adventurers. For whatever reason, many warriors take such stories as challenges to be bested, and the mysterious lairs of these monsters are constantly sought.

Lairs are areas in the game which take a bit of exploration to discover. They are usually populated by high level monsters, unless located within an RvR Lake, in which case they will be slightly above the lake's maximum rank.