Custom robots.txt is a way for you to instruct the search engine that you don’t want it to crawl certain pages of your blog (“crawl” means that crawlers, like Googlebot, go through your content, and index it so that other people can find it when they search for it). For example, let’s say there are parts of your blog that have information you would rather not promote, either for personal reasons or because it doesn’t represent the general theme of your blog -- this is where you can clarify these restrictions.
However, keep in mind that other sites may have linked to the pages that you’ve decided to restrict. Further, Google may index your page if we discover it by following a link from someone else's site. To display it in search results, Google will need to display a title of some kind and because we won't have access to any of your page content, we will rely on off-page content such as anchor text from other sites. (To truly block a URL from being indexed, you can use meta tags.)
To exclude certain content from being searched, go to Settings | Search Preferences and click Edit next to "Custom robots.txt." Enter the content which you would like web robots to ignore. For example: