Jump is a Statcast metric that shows which players have the fastest reactions and most direct routes in the outfield. It's defined as: "How many feet did he cover in the right direction in the first three seconds after pitch release?"
The Jump leaderboards may be viewed at Baseball Savant here.
In addition, Jump can be broken down into three components.
• Reaction: Feet covered (in any direction) in the first 1.5 seconds. Think of it like a way to measure first step.
• Burst: Feet covered (in any direction) in the second 1.5 seconds. Think of it like a way to measure acceleration.
• Route: Compares feet covered in any direction to feet covered in the correct direction over the full three seconds.
All of them are measured in "raw feet" but reported in "feet above or below Major League average," for context. ("Average" being slightly different for each play, as each type of play has its own slightly different baseline, based on running back or not, near the wall or not, and the length of time the ball is in the air.) (Note that for seasonal leaderboards, only plays with a 90 percent Catch Probability or harder will be considered; on 'easy' plays, there's often no jump or direct route even required.)