As the name suggests an aluminium sailing boat is one constructed from aluminium (also know as aluminum in the United States). Aluminium is a stronger metal than the same weight of steel, so aluminium boats can be lighter in weight whilst still being strong, which is an advantage for sailing boats, especially those used for racing where lower weight equals more speed.
There are a few disadvantages too though, for example, aluminium does corrode, and more so in saltwater, so aluminium boats need repainting every couple of years and you will need a sacrificial anode.