Wait is a good way to keep from being trampled when you open the door to let your dog out. It's not the same as Stay. Stay means don't move. Wait means don't go any further.
Relationship based training philosophy

With your dog on leash, go to the closed door.
Ask your dog to Sit.
Say, "Wait" as you reach for the door handle.
The dog will usually pop up at this gesture, ready to go outside. Take your hand off the doorway and put him back into the Sit.
Repeat steps 3 and 4 until he holds the Sit when you reach for the door handle.
Crack the door open.
If he pops up, close the door and put him back into the Sit.
Repeat steps 6 and 7 until he holds the Sit while you crack open the door.
Gradually increase how wide you open the door, closing it as necessary and putting him back into the Sit.
Once you are able to open the door all the way while he Waits in the Sit, give your release command and let him out.
Practice a session daily until he has it. Then, you can try it without the leash.
Once he's got Wait down while Sitting without a leash, you can try it without asking for a Sit first.

Once he learns Wait for going out doors, it will easily translate to waiting before jumping into the car or waiting at a curb before crossing a street - anytime you want him to stop forward progress until you give the cue to go ahead.