This extension adds a new filter in the layered navigation with 2 option i.e in stock and out of stock. This way the user can filter only in stock products or out of stock products whatever they wants if your shop do have outof stock product this extension can be usefull for you.
URL param name : When filtering by stock status or any other filter a new parameter is added to the url From here you can change that parameter s name in case in-stock is conflicting with an other attribute.
Filter label : You can change from here, and translate the label of the filter.
Enabled for Search pages : This enables/disables the stock filter for quick search and advanced search results.