Have you ever seen someone cook fish and remove those tiny bones or the skin? That's a bit like what fillet means. People often use fillet to remove something from what they're working with.
However, in the world of redaction or removing parts of a document, fillet means something a bit different. It's like erasing or crossing out a part of a drawing or text that you don't want other people to see.
Imagine you drew a picture but you don't like one little part of it, like maybe you drew a tree but then changed your mind and wanted a person instead. So you use an eraser to remove the parts of the drawing that you don't want to see anymore. That's a bit like how fillet works but instead of an eraser, people use special software to hide or remove certain parts of a document or image.
So, in summary, fillet means removing specific parts from a document, like a fisherman removing the bones or skin from fish, or like an artist erasing parts of a drawing they don't want.