Opacity
Opacity is the state of being opaque. It's very hard to explain in my opinion, but I'll try my best. It's basically having a background image, and then having a faded image in front of it. The opaque layer will tint whatever the background is to what ever the opaque layer is. This is kinda confusing probably... Anyways, this is used in the common glossy effect, and probably some other things you've seen. I also like to think of it as a tinted window, with a background behind. I use it all the time when I make borders, or when I'm making affiliation buttons. It helps a lot. I am, obviously, going to show you how to do this with GIMP. I'll show you it with layers, but you can change the opacity with the pencil, brush, and other tools as well. But... I'm only going to show you how to do it with layers.
I also like to think of opacity as force. The force that the opaque area (in this case front layer) is coming at you. The higher the opacity, the more the image will show up. If the opacity is lower, than the image won't show up as much. In GIMP, opacity is measured in percents (it might not be, but it's out of 100, so it's good for me).
To give you an idea, here is an example of a high opacity. This image was made with two layers. The background layer was just that dark gray at the bottom. The opaque layer was just a white layer (white only on the top half of the layer, the rest is transparent on that layer), with the opacity set high.
Here, the opacity is set low. All the colors in the two layers are the same. You would find the same grey/gray and same white in the two layers on the this image. The only thing differentiating this and the one above is that the opacity for the opaque layer here is set low, so not as much force is being used.
Getting Ready
I take things so slowly don't I? Yeah well, that's just me. =]
To 'get ready', you must have the two layers you want, one a background, and one the layer you want opaque. Make sure on your layer list that the one that will be opaque is above the background layer. To make a new layer, click on the first button to the left below the layer list.
Here is the background image I'm going to use.
Here is the layer that will be opaque. The checkered bottom represents transparency, by the way. If I didn't put there there, you would see right through that part.
That was short enough, right?
Making it Opaque
This is the step.
Make sure that you are set on the layer that will be opaque, to well, make it opaque. If you're not, click on that layer on your layer list. It should be highlighted. That means GIMP is set to that layer. Right above the layer list thing, right above the actual list, there is an opacity slider (well there should be)... Yeah, move it around. The more to the right, the higher the force (or the opacity). All the way to the right, and you won't even see the layer. Move it around for fun... You like? Finally, find a good area that you like.
So, I did it with my two layers. Here are some tries of mine that I didn't like, only because the opacity percent didn't make the image what I was looking for.
In this image, I thought the opacity was set to way to much on the high side. I set it to 75%. I can barely see the see through of the image.
This image's opacity for the top layer wasn't set quite high enough. I can see I slight difference, but not much. This opacity was set to 15%.
Finishing Up
Finding the right percent for the opacity isn't that hard. You just slide a bar and stop when you like the results.
I set the opacity to 30%, and I liked the result. Here it is.
Obviously, the button isn't done. But... The opacity part is, and that's what matters on this page. =P