Dog boundary training is a type of obedience training where you teach your dog the "limits" of where he can and cannot go. For instance, you may live in an area that does not permit a fenced yard. In this case boundary training your dog can teach him that he's to remain inside the confines of the yard - even when it's not fenced. This helps to keep your dog safe... once he knows the limits, he is less likely to run out into the road and possibly get hit by a car or get into other trouble. However, be aware that dog training should never be considered 100% reliable (even if your dog acts like it is): you never know when something is going to prove to be too tempting for your dog.
Ideally, of course, your dog would have a fenced yard to call his own. If that's not possible, there are still other options like erecting an outdoor dog run. Boundary training is a fine idea all on its own, but remember that even when your dog appears to reliable, there are always temptations that could make him stray - people, other dogs, cars, or wildlife are common temptations. It's best to watch over your dog and ideally, you would be outside with him whenever you can. Doing so will give you more peace of mind knowing that your dog is safe.