As you can imagine of a country that is spread over more than 7,000 islands, many people in the Philippines aren't hooked up to the power grid, so a common light source is the relatively dangerous kerosene lamp. In addition to the fire risk, getting the kerosene is a chore that takes a lot more work than flipping a switch. Or, now, scooping up a bucket of seawater. Just put a cup of ocean in the bottom of the SALt lamp and it lights up.
Designer Aisa Mijeno's lamp (SALt is a contraction of Sustainable Alternative Living) uses the salt water as the electrolyte in a galvanic cell battery. In a simple battery—like the one in your car—two metal electrodes are dipped into an electrolyte solution. In this case, the electrolyte is salt water. Usually, these electrode rods are made from different metals, each of which reacts differently to the electrolyte: one gives up its electrons to the seawater, and the other collects them. If you connect the two with a wire, this movement of electrons from one to the other causes a current to flow in that wire. Put a lightbulb in the circuit and you're good to go. If you ever made a battery by sticking two wires in a lemon, it's a similar principle.
The energy comes from one of the electrodes, which is eventually spent, like a log on a fire will eventually be spent. Mijeno's says his SALt lamp will run for eight hours per day for six months before you need to replace the anode, and that to run it for those eight hours you just need a glass of water and two teaspoons of salt. There's even a USB port on the side to charge a cell phone or what have you.
Using such a plentiful resource for his lamp is a fantastic idea. It's hard to imagine living without something as basic as on-demand artificial light. The SALt lamp will eventually be made available for anyone to order, but it seems like the small-scale project is prioritizing local supply right now.