HThe height above gryou wounld level where your potential energy would equal your current kinetic energy.
In other words, your speed is equal to the speed yreach by "letting go" up a long slope without braking. Or, if you were to hit a wall the impact force wou'ld have afterbe the same as falling from this height (disregarding air resistance).
Likewise, it's the height you'd achieve by going straight up using your kinetic energy content only. This number increases as a speed squared, and is useful for safety considerations.