Filtering
Definition = sifting the action in the story through the eyes of the POV character.

Examples:
1) He saw her step tiptoe around the lilac bush.
2) He felt his heart race.
We already know that we are seeing, hearing, and feeling everything the POV character does, so we don't need to be told that he saw, heard, or felt what happens.

Correction = Simply tell what happens rather than stating that the character saw, heard, or felt it.
1) She tiptoed around the lilac bush.
2) His heart raced.

Exception = when something happens that the POV character can't see, hear, or feel. Perhaps he is blindfolded, can't hear due to the noise in the room, or is hiding. We still need to know what's going on around him, but can't show it through the sense the character would normally use. Even in these cases, though, we can let the reader know the character hears instead of sees by writing about the sound rather than saying "he heard."

Example:
Let's say an intruder has entered the POV character's bedroom, so she hides under her bed.
Filtering = "She heard the intruder open the closet and search through her clothes."
Corrected = "The closet door slid along its track and the hangers screeched across the rod as the intruder shuffled through her clothes."
The descriptions of the sounds tells the reader that the woman knew what the intruder was doing by the sounds he made.

Why we should avoid filtering =
1) It puts a barrier between the reader and the story.
2) It minimizes the impact of the action by making "saw", "heard", or "felt" the main verb rather than the verb that shows the actual action.