The good people at Pro Football Focus spend enormous amounts of time breaking down every player’s performance on every individual play throughout the season. In the end, players can then be given a final rating somewhere between zero (poor) and 100 (elite). If you want to learn more about their methodology, you can read PFF’s Player Grade overview.
Most of us as fans view PFF ratings this way: “If the grade fits my opinion of a player then it’s credible, but if the grade conflicts with my conclusions then PFF is stupid garbage and should never be trusted.