A premises is a piece of land, plus all the buildings and permanent structures attached to that land.
It’s not just any piece of land, though. A premises is a piece of land that’s recognized in some official capacity and has specific boundaries. Think of a piece of real estate, like a house sitting on a city lot. That lot, including the building on it, is the premises.
Anything else that’s fixed to the property, like outbuildings, gazebos, or pools, would be considered part of the premises as well. Moveable stuff, like furniture or people, would be on the premises, but not part of it.