Lairs were an area in Skylanders: Universe where your collection of Skylanders, Magic Items, your achievements, and other things were stored. They were located on your Skyland. Different designs for its entrnace could be unlocked for each of the Elements if the player had a Skylander from it in their collection.
It is often asked how one acquires their own Lair. Professor P. Grungally once theorized that it required exact planetary alignment at an exact point in time while wearing a boot made by a rare breed of trolls without feet. Later it was discovered that every Portal Master already has a Lair located on their own personal Skyland and that none of the above is actually needed.