Layers can be thought of as clear pieces of acetate stacked on top of one another. Some layers are solid, or opaque. Some layers may have transparency. If a layer is partially transparent, or uses certain blending modes, it will allow you to see through its contents to the layers, or objects, below.
When the Move tool is selected and you are working on an unlocked layer, pressing a single number on the keyboard will change its opacity. You need to type quickly to apply two-digit values such as 53 for 53% opacity.
Press 1 for 10% opacity.
Press 5 for 50% opacity.
Press 0 for ...