No mention of external dimensions, so I am going to assume that it exists in the abstract since you mention teleportation. Like you just think about it and teleport there, not carry it upon your person like a portable hole in D&D.
Like others mentioned, a living space of some kind. I’d worry at first where I teleport inside, like hopefully not inside my mattress. Also, when I teleport back out, where is that? Where I teleported in? What if that gets replaced, like say I popped out while in an empty parking garage, but popped back in and a car is now parked where I left off? Might be nice if I pop back out anywhere I choose. Pop in while in New York, pop out in London.
Man, I’d save so much on rent. Or at least storage.