Two materials (often n-type and p-type semiconductors) are joined at two junctions, one junction is heated and the other cooled. The temperature difference makes charge diffuse from the hot side toward the cold side, and this charge is what turns into the seebeck voltage they describe. It was just very hard to get anything meaningful out of this because you can't easily get such a temperature difference. If you've read of the peltier effect, it's the same thing as this, just in reverse.