The concept of filtering words was a concept that I think I instinctively was aware of, but never realized in detail until someone called it out to me. That person was someone from on the WotF forum, and in a critique swap, he pointed me to this article.
The article defines filtering as:
[…] unnecessary words that separate the reader from the story’s action. They come between the reader’s experience and the character’s point of view.
Since I’m already leaning on this article, I’ll continue by borrowing a bit of their example.
Mary felt a sinking feeling as she sped across the room to yank aside the curtains. She wondered if her husband would really leave.
Mary’s stomach sank as she sped across the room to yank aside the curtains. Would her husband truly go?
The difference is in how the reader experiences the events being conveyed. In this situation we’re dealing with a tight 3rd person POV. In the example, we’re inside Mary’s head, experiencing her husband leaving through her eyes, through her feelings. We know she feels the sinking feeling simply on account of her telling us about it.
Filtering words are often verbs that describe what’s happening to a POV character, see, feel, hear, think, seem, notice, etc… and when in the sausage-making phase of putting a first draft together, I find for myself that they weasel their way into my prose with an alarming frequency. Luckily the wizards who wrote your favorite word processing software invented Ctrl-F, and every one of these can be fixed in post.
That list of Ctrl-F search terms grows, but the final product improves because of it.