http://***.com/q/211383/1317805\">清除 float 属性您的 nav 已通过其 .col 类获得。否则，您的 nav 元素之后的内容可能会出现在您的 nav 元素旁边而不是下面。

It's a method used to clear the float property your nav has obtained through its .col class. Without this the content following your nav element may appear alongside your nav element rather than below it.

有关进一步阅读，请参阅Chris Coyier的清除浮动的方式和原因。

For further reading, see Chris Coyier's The How and Why of Clearing Floats.