estat abond

Arellano-Bond test for zero autocorrelation in first-differenced errors
  +-----------------------+
  |Order |  z     Prob > z|
  |------+----------------|
  |   1  |-3.0656  0.0022 |
  |   2  |-2.8556  0.0043 |
  +-----------------------+
   H0: no autocorrelation
请教一下各位大神，这是什么意思啊