Filter words are those extraneous words writers use that shield readers from the action. The use of filters lends to telling not showing, the biggest of all writer no-nos. When filters are used, the reader sees everything from the characters point of view, instead of experiencing the action themselves. Many call this a deeper POV.
For instance – Paul felt his heart thud when Aaron entered the room.
Felt acts as the filter. Of course he felt his heart. Who wouldn’t?