The order of animation layers in the Animation Layer Editor stack is important because it can affect how the result animation is calculated in your scene. See Animation layer stack for more information on how the order of layers can affect the result.
As you drag, a black line or a rectangular outline appears to indicate where the layer will be placed. If the layer will be placed between two layers, a single black line displays. If the layer will be parented, a rectangular outline displays around the layer that will become its parent.