在stata的reference中写到
“A problem with the original Arellano-Bond estimator is that lagged levels are poor instruments for first differences if the variables are close to a random walk.  Arellano and Bover (1995) describe how, if the original equation in levels is added to the system, additional instruments can be brought to bear to increase efficiency.  In this equation, variables in levels are instrumented with suitable lags of their own first differences. ”

这是说当变量为随机游走过程时，lagged levels不是一阶差分项的好工具变量。所以提出增加附加的工具变量以提高估计精度（这种为系统GMM），通过利用水平滞后项作为一阶差分项的工具变量。
问题：
其中lagged levels是指什么，是一阶差分项的滞后吗？
从差分GMM发展到系统GMM，原理最直白的解释是什么？
希望得到解答。