"Filtering" refers to phrases like "I saw", "he feels", "she heard", etc. It distances your reader from the world, by forcing them to look at the character looking at the thing you're describing.
Instead, describe the thing the character's seeing/hearing/feeling. We'll catch on. I promise.
Source: http://jwtroemner.wordpress.com/2013/04/17/diy-editing-changing-the-filter