What Does It Mean When Ears Lay Back?

This can be a sign of fearfulness. Sometimes if a dog has been hit in the past, then they will lay their ears back when you try to pet them because they are flinching and waiting for the slap. Some dogs will lay their ears back simply because they are very timid or scared. In some cases a dog will lay their ears back and show their teeth to defend themselves if they feel threatened. Consider the situation to know why the dog is responding this way. Regardless, this is a sign that something is affecting the dog to make him respond this way.

My Pit does this when she hears a noise sometimes. Like she is trying to hear something far off.

As Fishgirl mentions, I think it may be an instinctive way of protecting the ears, when threatened,  which are a vulnerable part of the body.