So in order for a guitar to resonate, the top (especially) and back plates have to be able to vibrate... so they're very thin. But in order to support all the pressures (particularly of the strings) that bear on those plates, they need reinforcement. That's where bracing comes in.
The braces serve several purposes... one is that they keep the plates from caving in. Another is that the placement and size and shape of the braces affect tone. Another is that they help create the structure of the instrument. For structural and tonal reasons, most guitars (even the ones that look flat) have a radius (convex curve) built into both the top and the back. The way that's done is to radius the braces, and then the plate is glued to the curved braces.
So the braces start as a pile of what looks like kindling... spruce, which is light and strong. And the braces have to be cut to the right length, and then radiused to the proper arc. There is a nifty jig for doing this on the big upright belt sander (which I'm pretty sure has a different name, but there it is in the picture).
After that is done, the braces are shaped so that they're wide on that curved surface where they will meet the plate, and tapered to a peak. This is tricky to do, and is done on the flat/horizontal belt sander... but b/c the pieces get pretty thin, it's easy to sand off your fingernails (as I did... quick manicure!) or even nick your thumb (as several of my classmates did... they all had matching thumb bandaids today). So that's what you see in the third pic.
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