A filter is used to keep the coffee grains from funneling down into the coffee pot.
What kind of filter are you referring to? A filter on an email client, a server side filter, filtering out web content, filtering images, a filter to remove scenes from a movie, a key filter?
It really depends, but the generic is actually show above with the coffee filter analogy. A filter is used to keep something out or only let certain things through.
The admins role would be to administer the filter, get it working and integrated with the system and maintain it as necessary. Generically speaking.