The Product Filter module control is optionally used beside the product list module control to refine results. The filter works against product attribute definitions assigned to a product or variant. The product attribute definition (Catalog > Attributes definitions) must be marked as "Filterable" in order to be listed in the filter. Currently, only Boolean, Decimal, Integer and Selection attribute definition types can be filtered. See for more info. You can also filter against certain core product properties such as the price, manufacturer and distributor.