A pre-filter is a sheath that covers the exterior of your active carbon filter. It protects the filter from larger particles and thus prolongs the life of the filter. You should always use a pre-filter with your Prima Klima filter.
Pre-filters do need replacing when they become clogged with particles. How often you replace your pre-filter is somewhat dependent on the environment in which you are using it. In a very clean environment you should get up to 12 months of use out of a pre-filter. However if you are using it in a very dusty environment it may need changing as regularly as once ever 6 months.
You can purchase new Prima Klima pre-filters from your usual retailer. All UK Prima Klima retailers are listed here.