The Exit velocity is the speed at which a batted ball flies away from the bat. It can be measured by advanced tracking systems such as Statcast and helps to determine whether a ball was hit well or not, and whether a particular outcome - a hit or a homer - was a matter of skill or luck. As a general rule, the higher the exit velocity (i.e. the harder a ball is hit) the more likely this is to result in a positive outcome for the offense. Thus, batters who constantly hit the ball hard are highly valued, even if their results might be mediocre in the immediate; conversely batters who maintain a high batting average in spite of not hitting the ball hard are generally considered lucky and unlikely to maintain their level of performance in the future.
The measure can also be used to evaluate pitchers: those who allow a high number of hard-hit balls are unlikely to have long-term success. The measure can also be used by official scorers to justify a decision between a hit or an error (hard-hit balls are also harder to field properly).