Some dogs need space  
 
If you see a dog with a yellow ribbon on the leash this is a dog who needs some space. Please do not approach this dog or its people with your dog. They are indicating that their dog cannot be near other dogs. How close is too close? Only the dog or his people know, so maintain distance or give them time to move out of your way.

There are many reasons why a dog may need space: 

Maybe he has health issues or is in training.
She may be a rescue dog being rehabilitated. The world can be a very scary place for these dogs. 
He may have had a bad experience with another dog or just not like the kind of friendly dogs who always want to say “Hi!”

In short, a yellow marker on a dog means it needs a little space.

Thank you! Those of us who own these
dogs appreciate your help and respect.

(info taken from: http://www.gulahund.se/default_en.asp )