There’s no shame in asking for a refresher on a canon thing ;] While I can’t give you the full history (check out the Vampire Companion for that), in brief: 

Night Island was built by the vampire Armand as a gift to his mortal lover Daniel Molloy, to share w/ Daniel, and it is vividly described in Queen of the Damned. I gather that it is in fact a complex built on a man-made island (which does not exist IRL sorries) off the shore of Miami, Florida, USA. Daniel describes it as such:

“And Armand, he was probably roaming the dimly lighted rooms of the villa, steps away from the tourists and the shoppers, yet utterly cut off by steel doors and white walls – a sprawling palace of floor-length windows and broad balconies, perched over white sand. Solitary, yet near to the endless commotion, its vast living room facing the twinkling lights of the Miami shore.”

“… the Night Island, Armand’s own personal creation with its five dazzling glass stories of theaters, restaurants, and shops.”

[^This will be yer face if you visit there]

It’s meant to be a kind of universe of funtimes, where mortals can enjoy all kinds of entertainment, so it’s a big source of revenue for Armand, but more importantly, it’s where the coven gather and bond over a short period of time after a really harrowing shared adventure.