Layer masks are a great asset to Photoshop and can open up an entire new world of photo editing – once you know how to use them. If you already understand the concept of layers, you know that layers can be adjusted in transparency – referred to as the opacity level in Photoshop. This makes the top layer invisible to a certain extent and allows the bottom layer to be seen. This is adjusted using a percentage rate – 90% opacity would be barely see-through, while 10% opacity would make the layer virtually invisible.