It is basically a bit of code that check whether your visitor is using a mobile device or not, if so it uses a cut-down theme, with extra navigational links to make it easier to browse without a mouse, and cuts down on what is xfered to the device, resulting in lower bandwidth needs for the visitor – so cheaper and quicker.

It is free, I’m not the author, just a satisfied user…
(My blog is also on making money (occasionally), Kazakhstan (quite often) and random stuff (very often ))