Route Efficiency is determined by dividing the length of the "optimal route" for a fielder by the length of his actual route to the ball. The ground a fielder covers before fielding a ball is known as his "Distance Covered." Obviously, any fielder would like to have his Distance Covered be as close as possible to the length of his optimal route to the ball.
The optimal route is determined by the straight line from the fielder's position at contact to the point where he fields the ball.
A fielder's goal is to have his Route Efficiency as close to 1 as possible. (Route Efficiency is expressed as a percentage, meaning a fielder wants his route to be as close to 100 percent as possible.)
However, there may be times when a 100 percent Route Efficiency isn't optimal based on game situation. Outfielders sometimes prioritize getting behind the baseball to make a throw, slightly hindering their Route Efficiency. And when a ball is hit near the wall, outfielders are taught to find the wall first, before making an adjustment if necessary.
Still, every outfielder wants to take the most direct route possible to a batted ball, and Route Efficiency measures exactly how well an outfielder does that.
Watch: Statcast tracks Mike Trout's Route Efficiency on a catch in center field.