Scumbling is a painting technique for adding a layer of broken, speckled, scratchy color over another color. Bits of the lower layer(s) of color show through the scumbling. The result gives a sense of depth and color variation to an area.
Scumbling can be done with opaque or transparent colors, but the effect is greater with an opaque color and with a light color over a dark. When you look at it from a distance the colors mix optically. Up close you'll see the brushwork and texture in the scumbled layer.
You can scumble with a brush or a crumpled-up cloth (if you've ever done decorating paint effects, you'll recognize it's a bit like sponge-painting a wall, on a small scale). The key is to use a dry brush (or cloth) and very little paint. It's far better to have to go over an area again than start with too much paint.
Dip your dry brush into a bit of paint, then dab it on a cloth to remove most of the paint. It helps if the paint is stiff rather than fluid, because it doesn't spread as easily when you put brush to canvas. Try to keep the brush hairs relatively dry, rather than soaking up moisture from fluid paint. If your brush is very moist, hold a cloth around the hairs at the ferrule end rather than at the toe. This will help pull moisture out of the brush without removing the pigment.
Think of the technique as rubbing the last little bits of paint from the brush onto the painting, leaving behind fragments of color. (Or if you like being vigorous, think of it as scrubbing at a painting with a not-quite-clean brush.) You're working on the very top surface of the painting, the top ridges of the paint or the tops of the canvas fibers. You're not trying to fill in every little piece.
Don't use your best brushes for scumbling as you'll most likely push hard on it and flatted the hairs at some stage. Either buy a cheap, stiff-hair brush that you sacrifice for scumbling, or use an old, worn-out one.