A white umbrella, maybe the most used, as a shoot through. As it is made of a thin, translucent white material, shooting through an umbrella of this kind will cause the light to be softened and smooth.
The normal practice is that you attach the umbrella to the stand, in the bracket underneath the strobe in a way, so the strobe will fire into the umbrella, and will hit the subject softer, and not as harsh as from a strobe directly, or a silver umbrella. The umbrella looks just like a normal umbrella that you use in rainy weather. Just without a handle. It folds up the same way. This also means, that such an umbrella, for studio photography, is a relatively cheap accessory, for your home studio. You can also use the white umbrella, for bouncing the light. This way, it does not give as much light as a shoot through, or as much as a silver umbrella will reflect, but it will be a much softer light to hit your subject.