Attribution window is a timeframe for your conversion reports taken as a result of your ad. Facebook tracks the actions of your ad during a particular period of time. Post click and post view cookie time options are 1, 7, 28 days. By default, Facebook shows 1 day post view and 28 day post click metrics. You can adjust this settings on "Reports" interface or in "Metrics" tab under each of your campaigns by clicking on the Attribution Window button and choosing from the dropdown menu.
You can have the detailed information about attribution window by clicking here.