Blocker

A so-called blocker on a website blocks search engines, so they can’t access and index it. As a result the website won’t be displayed in the search results.

Example

If the complete website is password protected, search engine crawlers can’t get in.

When possible, lockers should be avoided because the purpose of SEO is to reach top positions in the search results.

Of course there can be various reasons why some pages shouldn’t be indexed. But those pages don’t need to be optimized for search engines.