Torpor is the state of deep, hibernation-like sleep that Vampires enter into from time to time. Sometimes it is a voluntary thing, when they are bored with unlife and want to wake up in a new era, or to hide from an enemy, or to wait for everyone to forget the stories of dozens of missing young women. Sometimes it is involuntary, as when they are sufficiently injured to kill a mere mortal, or starving for lack of fresh blood, or simply so old that they go to sleep one morning and forget to wake up the following night. Torpor can last for hours or centuries, depending on how and why it was entered.
Staking a vampire, piercing their chest cavity with a sharp wooden object, will instantly send them into involuntary torpor until the stake is removed...at which point they wake up, instantly if its been mere moments since they were impaled, or groggily over several hours if it's been a century or more. Staked vampires can be stored in this way for long periods of time, which can make gathering an army of them surprisingly affordable.
Voluntary torpor is traditionally done in a coffin or other similar container, easy to conceal by burial or interrment inside the walls of a building, but can be done in any environement. Involuntary torpor tends to happen wherever you got staked or fell over unconcsious. Direct sunlight will rouse any vampire from torpor but this is generally a fatal experience (either for the vampire or whatever idiot opened the drapes).