"This is Milliways, it's a place between worlds. Sometimes, a door will lead here, instead of wherever it's supposed to lead. People from any time or place you can imagine end up here, and so long as you're in this place you can understand all of them. The door will take you back home, if you want to go, or if someone else opens it for you it will take you to their world. Once you close it, though, you can't get back until Milliways steals another door you're going through - and many people who find this place only ever find it once."
As for where, "Bar over there," she motions towards the bar, "Is a person. She will sell you almost anything that isn't magic, a weapon, or a living creature, and she also rents out rooms. If you want somewhere private, my brother and I will get one for you. Or, if you don't want us to rent you a room, you could also go outside," she motions to another door.