What is the question mark behind the externally introduced CSS file name?

What is the question mark behind the externally introduced CSS file name:

When you look at the code of the page, you sometimes find that the external CSS file name introduced by the <link> tag has a question mark after it, and the question mark is usually followed by some random eight bad characters, this site is also the following:

Perhaps people think that the character behind the question mark has a special meaning, it is not so, can be modified to any character, it is like a version of this CSS file, for example, when the Web page has been visited, may have the original CSS style in the cache, although the server's CSS file may have been updated, However, the browser can not be updated, if you add a question mark and the following characters, you can update the CSS style page.

The original address is: http://www.51texiao.cn/div_cssjiaocheng/2015/0504/722.html

The original address is: http://www.softwhy.com/forum.php?mod=viewthread&tid=4642

What is the question mark behind the externally introduced CSS file name?