Use settle whenever you are in public and want your dog to stay quietly beside you, usually in a down/stay.
To train this, put your dog in a down and ask him to stay. It may only be for a nanosecond at first but be sure and reward before he gets up. Put your leash on your dog. Now move to a chair and ask your dog to down and stay and step on the leash so your dog can still be comfortable but cannot get up. Randomly drop treats and tell him “yes” or “good” but make sure the dog can reach them without any tension on the leash. Keep the leash tight enough so he can’t get up. Start with a short duration and increase. If your dog struggles at any time, take your foot off the leash and go back to just asking for a down/stay. Eventually you’ll get a nice relaxed down/stay. Now you can add the word “settle”.