Teaching the place command can be used for many purposes, but is great to use when you have guest come over to your house. If your dog tends to greet guests with too much enthusiasm, I would recommend teaching and enforcing the place command.
The place command is having a specific place for your dog (bed, mat, rug) to go lay down in and stay until he is released. It is important for this place to be the same throughout your dogs training, and for it to be easily recognizable for your dog.
To start off, stand with your dog next to the designated place and hover your hand above the place until your dog steps onto it. Click and drop the treat onto the mat/bed/etc. for your dog to get. Repeat this until your dog knows to get onto his spot with just a soft hand movement.
Next, tell your dog to lie down when he steps onto his mat, and don't treat until he does so. After a few training sessions like this, your dog will begin to lie down without you asking.
Now you can start taking small steps back and repeating the above steps until you are sending your dog multiple feet away from you to go lie on his place. Go slowly and set your dog up for success.
Once your dog gets the hang of what you are asking of him, you can begin to say the command long distance, such as in another room or from across the room. Part of the place command is for your dog to stay on his mat until released, teach this the same way you would teach stay but without an initial command. Treat your dog for staying on the mat, correct for getting up, and slowly increase the three D's (Distance, Distraction, Duration) until your dog goes to his mat and stays even when strangers are walking through your door.