Displacement behaviour is an action that seems out of context for the situation a dog is in, often appearing when the dog is experiencing some kind of internal conflict or mild stress. Common examples include scratching, shaking off as if wet, sniffing the ground, or suddenly grooming themselves, none of which relate directly to what's actually happening around them.
The behaviour itself is normal. What makes it a displacement behaviour is that it appears at a moment when it doesn't quite fit, suggesting the dog is redirecting some internal tension into another action.