"Just a regular key."
The Back Door Key is a key item in the Resident Evil 7 Teaser Demo: Beginning Hour.
The Back Door Key is used to unlock the back door. Proceeding through the door leads to the "Welcome to the Family" ending.
The Back Door Key is located in the secret room near the fireplace at the foot of the hole. It only spawns there after having completed watching the "Derelict House Footage" tape.