I find the phrase 韬光养晦 in a context,and I look up it in the dictionary,it means hidden talents, do not make the leakage.But I still don't know the meaning of 晦.
晦's original meaning is dark moonless night, here it figuratively means the lack of accomplishment or culture. And other words' meanings are:
韬 : restrain, hide
光 : shine, ray of light, here it figuratively means talent
养 : cultivate, train
So 韬光养晦 means to hide talent and keep improving, to achieve balanced development.
The literal meaning of '晦' is the dark night, without the moonlight, as @songyuanyao said. and in this idiom, '晦' means one's shortcoming, weakness, opposite to strong and advantage.
So you can comprehend this idiom in this way: In order to survive, one needs to hide his talent and pretend to be weak, so that he can get enough time to be strong and defeat his opponent.