F-packs are usually used post fermentation and stabilization to sweeten and sometimes flavour the wine. There are three types of F-packs in the wine kit world. For a mist wine (eg Blackberry Merlot), the F-pack contains the fruit flavouring and the sweetening. For an off-dry wine (eg zinfandel blush), the F-pack just contains sweetener. For specialty wines (eq port, sherry, ice wine), depending on the manufacturer, there can be multiple packages to add, some with flavouring, the rest with sweetening.
Steve