Defense is a very difficult thing to understand when it comes to baseball. Who is good? Who is bad? Historically we used metrics like fielding percentage to rate our fielders. But fielding percentage does not take into account all plays that weren’t made.
Over the last decade or two, the best publicly available metrics to measure defense were DRS (Defensive Runs Saved) and UZR(Ultimate Zone Rating). Both used a system of dividing the field into zones and creating a debit or credit system for plays normally made within each zone. But neither could account for shifts or positioning prior to the play.