From the description this sounds like they are using a form of what is commonly called a lemon or potato battery (http://en.wikipedia.org/wiki/Lemon_battery). The salt water is the the electrolyte in the battery cell and the actual energy comes from a chemical reaction between the zinc and copper electrodes.
In this blog post (https://saltph.wordpress.com/2014/05/14/know-more-about-salt...) they mention that part of their revenue model is "Electrode rod refill", and in their main page (http://www.salt.ph/), under "Our Product" / "Just add water and salt!" they mention replacing the anode every 6 months. The anode they are referring to would be the zinc electrode. This is usually a galvanized nail in those popular lemon / potato battery science experiments.
In this blog post (https://saltph.wordpress.com/2014/05/14/know-more-about-salt...) they mention that part of their revenue model is "Electrode rod refill", and in their main page (http://www.salt.ph/), under "Our Product" / "Just add water and salt!" they mention replacing the anode every 6 months. The anode they are referring to would be the zinc electrode. This is usually a galvanized nail in those popular lemon / potato battery science experiments.