This dog is worried. Worried dogs may bit at the unfamiliar.
Even the cuddliest, fuzziest, sweetest pup can bite if provoked. Most people are bitten by their own dog or one they know. Some owners actually promote aggression in their dogs or allow aggression to go unchecked. http://www.avma.org/animal_health/brochu...
Did you know that 50 percent of all children in the United States will be bitten by a dog before their 12th birthday? Did you know that 800,000 bites a year are severe enough to require medical treatment, while 1 to 2 million go unreported? The vast majority of dog bites are from a dog known to the child--his or her own pet, a neighbor's or friend's. http://www.aspca.org/pet-care/dog-care/d...
How many people don't believe dogs speak? Well, once you know what to look for you may change your mind. It's more than just barking. Dogs have an intricate body language that we may only guess at. Between vocalization and language we might have a clue about how dogs talk. Dogs have a variety of barks that vary from happy and high pitched to excited and frightened, to the alert bark (alerting you to someone approaching your home, your territory).
There is much more information than I can fit into one blog so I'll try to sum it up for you.
Never approach a dog that is not wagging a tail, have their teeth bared, don't have a worried look on their face, doesn't have their tail tucked between their legs, cowering in a corner, not sitting peacefully, that doesn't have an owner nearby. One of the worst things to do would be to make eye to eye contact for more than a few seconds. I have seen a few dogs who smile but they aren't very common. I'm not trying to make us all paranoid but safe dogs are not always safe.
Safe dogs should be, but not necessarily, ones that are relaxed or asking for play with their tail up or a paw extended.