texts = [[word for word in document.lower().split()] for document in documents]

我在网址我爱自然语言处理-如何计算两个文档的相似度（二）中看到下面一份代码。
对于>>> texts = [[word for word in document.lower().split()] for document in documents]的含义不是很理解。

>>>documents = ["Shipment of gold damaged in a fire", ... "Delivery of silver arrived in a silver truck", ... "Shipment of gold arrived in a truck"] >>> texts = [[word for word in document.lower().split()] for document in documents] >>> print texts [['shipment', 'of', 'gold', 'damaged', 'in', 'a', 'fire'], ['delivery', 'of', 'silver', 'arrived', 'in', 'a', 'silver', 'truck'], ['shipment', 'of', 'gold', 'arrived', 'in', 'a', 'truck']]

对于一般的for var in list:这种形式，我是知道的。但是上面的那种二维数组，我就不是很理解为什么了。求助，帮忙分析一下