I don't know how this plot was created but judging by the axis labels I can estimate what the storyline is. Below is a very general description that you can interpret under your NN's context.
It looks like something was tested many times (my estimate is ~700 iterations) and on each time the error was computed between Targes and Outputs (from the xlabel). All of the error was quite low and falls in the range of -1.1e07 to 3.29e08 but most of the error falls within the range of x-values where you see the high bars. The height of each bar is the number of tests (ie, 'instances').
Notice that the orange line "zero error" separates negative and positive ticks. The sign of the error shows you the direction of the bias. Positive error means the outputs were less than the target and negative error means that the outputs were greater than the target (assuming the targets are positive values) . I base this on the x-axis label that defines error as Targets-Outputs.
The majority of tests were negative so the marjority of outputs were slightly larger than the targets.
