A solid state relay (SSR) is just what it sounds like; an IC that acts like a mechanical relay. They allow you to control high-voltage AC loads from lower voltage DC control circuitry. Solid state relays, have several advantages over mechanical relays. One such advantage is that they can be switched by a much lower voltage and at a much lower current than most mechanical relays. Also, because there's no moving contacts, solid state relays can be switched much faster and for much longer periods without wearing out.
They accomplish this by using infrared light as the 'contact,' a solid-state relay is really just an IR LED and a phototriac sealed up into a little box. Thanks to the fact that the two sides of the relay are photo-coupled, you can rely on the same type of electrical isolation as in mechanical relays. These particular solid state relays can switch 400VAC and 8A.