首先你这整条语句的意思就是：调用一个外部的CSS样式文件。他是通过这个标签来调用的。
然后， href="css/index.css" 表示外部样式文件的路径，rel="stylesheet"表示调用的是一种样式。
最后，type="text/css" 就具体说明调用样式的文件类型为CSS样式！
好处：调用外部样式能达到内容与样式分离的效果，优化网站！

这个标签的rel属性用于设置对象和链接目的间的关系，说白了就是指明你链进来的对象是个什么东西的，具体的值及其所表示的关系如下：
Alternate:Substitute version of the file that contains the link.
Appendix:Page that is an appendix for the set of pages.
Bookmark:Bookmark.
Chapter:Page that is a chapter for a set of pages.
Contents:Table of contents document.
Copyright:Copyright notice for the current page.
Glossary:Glossary for the current page.
Help:Help document.
Index:Index document for the current page.
Next:Next document in a sequence.
Offline:href that contains a path to the CDF file to be used for an offline favorite.
Prev:Previous document in a sequence.
Section:Page that is a section for a set of pages.
Shortcut:Icon href that contains a path to an icon file to be used for the favorite or link.
Start:First document of a set.
Stylesheet:Style sheet.
Subsection:Page that is a subsection for a set of pages.