Give the [FAQ (frequently asked questions)][1] a read. 

There is a whole section explaining what reputation is, how to earn it, and earning it enables you to do.

> The primary way to gain reputation is by posting good questions and useful answers. Your peers will vote on your posts, and those votes will cause you to gain (or, in rare cases, lose) reputation:


  [1]: https://superuser.com/faq#reputation