Jump ability is the potential for jumping found with each character. As far as properties of jumping go, this often refers to the height or trajectory of the jump. Occasionally, this refers to its speed or duration.
If a jump goes high, then it will be easy to stay away from attacks like projectiles, but it becomes more difficult to adjust one’s position. If it goes low, it gets difficult to skip over the opponent, which makes jumping in safer, but contrastingly makes it tougher to aim for crossup.
For cases where the jump is too high, if you can use a small jump or a low-altitude air dash to mitigate the effect, it is possible to follow through.
Jump attacks generally have higher-priority attack hitboxes facing downward, so when meeting in midair, the higher jump becomes more effective in these cases (though exceptions abound).
The slower the velocity, the longer the flight duration; this makes one more susceptible to anti-air moves, so in general, slower jumps are less effective.
If there is an air block, and the opponent doesn’t have a non-air-blockable move, this restriction does not hold, but the fact that the slowness makes it hard to manage does not change. Also, on top of the velocity being slow, there is the height attribute; when it reaches an extreme of being hard to use, the delicacy of the situation can be likened to tissue paper (which is why some Japanese players refer to it as such).