Filters are used in various parts of the system. They work on new and already retrieved posts as well as on subscriptions. They allow you to select desired items out of hundereds or thousands of items, while filtering out unwanted ones.
Filtering can be divided into two major categories based on filtered item, that is either posts or subscriptions.
In case of filtering posts, filters check posts for occurances of special words and/or look for whether they posted in desired time interval. Post filters are used for various purposes, such sweeping/deleting unwanted posts and gathering them for virtual channels from other subscribed channels.
By default, Subscriptions Panel lists all your subscriptions so that you can open and manage them thru that panel. Same panel also allows you to filter listed subscriptions in order to see a subset of all your subscriptions. For example you can see only those subscriptions which were updated within last hour, thanks to filtering on subscriptions.
To learn more about where these filters run and how they are used, check out help topics listed below.