The next part of this tutorial covers Mode & Opacity. These two options combine to provide a decent amount of simple, yet powerful layer manipulation in GIMP.
Each of the many modes provide a different way to display the layer, and the opacity determines how transparent/visable the layer is. Below is a sampling of just a handful of the different modes and how they effect a layer.
Normal is the default mode for all layers. The above shows the yellow oval with a 50% opacity.
Again showed at 50% opacity, dissolve essentially dissolves the layer pixel by pixel with nearly all of the layer showing at 100% opacity, and nearly nothing showing as the opacity approaches 0%.
Multiply makes the layer feel almost like a lens filter and gives the effect of mixing colors.
Difference shows the color remaining after removing the layer’s color from the background.