It refers to the "lack of knowledge" when learning. You should always reflect on yourself and know your own shortcomings, so that you can not be complacent and eager to learn and make progress.

"Contented people are always happy, and ninjas can be complacent."

It means that people will always encounter many unpleasant things in their lives. The so-called "nine times out of ten", these things may not be changed by manpower, and forced changes may cost a lot. At this time, people should be "satisfied", accept imperfections, admit defects and know how to be satisfied in order to live happily.

So "lack of knowledge" means lack of knowledge, and "contentment" means satisfaction, and the two meanings are not in conflict. Moreover, "lack of knowledge" is used in learning, and "contentment is always happy" is used in life, and the scope of the two does not conflict. Therefore, we should not only "lack knowledge" but also "be satisfied", so as to be both "eager to learn" and "always happy".