What is a fliter? some definitions from the web

A program that accepts a certain type of data as input, transforms it in some manner, and then outputs the transformed data. For example, a program that sorts names is a filter because it accepts the names in unsorted order, sorts them, and then outputs the sorted names.Utilities that allow you to import or export data are also sometimes called filters.
A pattern through which data is passed. Only data that matches the pattern is allowed to pass through the filter.
In paint programs and image editors, a filter is an effect that can be applied to a bit map. Some filters mimic conventional photographic filters, but many transform images in unusual ways. A pointillism filter, for example, can make a digitized photograph look like a pointillistic painting.
Filters are applied to the information coming into your account, to manipulate the final data in order to provide accurate reports. These filters can be set up to exclude visits from particular IP addresses, to report only on a subdomain or directory, or to take dynamic page URLs and convert them into readable text strings.
In telecommunications, a filter is a device that selectively sorts signals and passes through a desired range of signals while suppressing the others. This kind of filter is used to suppress noise or to separate signals into bandwidth channels.
In Photoshop and other graphic applications, a filter is a particular effect that can be applied to an image or part of an image. Filters can be fairly simple effects used to mimic traditional photographic filters (which are pieces of colored glass or gelatine placed over the lens to absorb specific wavelengths of light) or they can be complex programs used to create painterly effects.