What are filters and how do you do them?

This topic was in here not so long ago..

As i remember Filters are a wash that you use to cover the entire model.
Washs you manly concentrate on bolts, panel lines etc. I was always doing this method for my AFV, but a concentrated wash for my Aircraft. So i didnt even know it was called a Filter lol, i just thought of it as a cover all wash, but now it has a name....so u know what it is

P.S i may be wrong so correct me if so, i've only just learned about this method also