The real short version of PFF is they what each snap and grade each player, broken down by type of play and responsibility (-2.0 through +2.0)--pass vs run play for linemen, etc. The responsibility part is key.
The casual fan might yell at the wrong player because they don't see enough of the play. The biggest example of this is zone coverage. One might yell and scream at Burnett because a deep-right throw is completed and he's the closest Packer, but really Bush slipped and fell early. Burnett then makes a super great play just getting close and minimizing the damage. His real responsibility was deep left.
For line play, passing a stunt off to the correct player might be the right play, but if Lang messes up the stunt, Newhouse might look bad.
Short version, higher numbers mean doing more things right. Last year, he was graded very poorly. This year, he's doing rather well.