The Twenty-Six Malignant Gates

In Joy Luck Club, The Twenty-Six Malignant Gates is a Chinese book detailing all of the dangers that could befall a child. It's basically like an older version of scare tactic after school specials—you know, the kind of PSAs that taught kids that if they drank beer/ate too much candy/skipped class/wore short skirts, they'd die—only geared towards moms.

So the book pretty clearly symbolizes a mother’s desire to protect her children against any and all dangers they face.

Part II of The Joy Luck Club begins with a little parable: a mother reads The Twenty-Six Malignant Gates and is worried her young daughter will have a horrific accident while riding her bike. The mother is wracked with fear because her copy of The Twenty-Six Malignant Gates is in Chinese, so the English-speaking daughter can't read it. 

This parable hammers home a couple of points. The first is a sort of mother-knows-best message: the fact that the book is in Chinese means the daughter has to trust the word of her mom. But the second is a little more nuanced: the fact that the book is in Chinese also underlines the communication errors that happen in any parent-child relationship. You can't always shield your kids from danger, period.

And both of these points are ones that the moms in The Joy Luck Club struggle with. They want to shield their daughters from the world, but a combination of lack of trust and failures in communication keeps getting in their way.