As I use it, it means the distance (linear distance) that I can hit a critter in the kill zone and still have a "depth" of field or range error/buffer. The further out one shoots the steeper the trajectory and the depth of the danger space decreases accordingly.
As an example, with a 308 and 175's zeroed at 700 yards the round is 8.2 inches high at 675 yards and 8.7 inches low at 725, this 50 yard area produces a total of 17 inches of drop in my round. If I make an error of 25 yards in ranging I'll miss the target/kill zone, I'd need to be albe to range to within about 12 yards of the actual range to get a good kill allowing only 4" up or down, that equates to a 25 yards danger space at the far end of the trajectory (same for the near end, I'd be 4.1 inches high at 25 yards).
Read on this site, they have a good explanation of danger space, it's hidden in the Point Blank Zero section.