While the Move Layer tool is selected, the Tool Weight Slider becomes the Layer Opacity Slider. This slider controls how opaque or transparent the current layer is.

When the slider is all the way to the right, the layer is fully opaque (the default). When it is all the way to the left, the layer is completely transparent,  or invisible.

You can animate a layer's opacity to have it fade in during a frame.

If layers are nested, child layers are affected by changes in their parent's opacity.  So, for example, nested layers can be faded in or out together.