Input field that launches a Confluence search.
The Confluence search box is located in the Confluence Header by default and is the interface for launching a site-wide search from anywhere in your site.
The Quick Search results that appear below the field will initially show you pages that you've recently visited. Once you start typing, the search engine will immediately start searching all pages, blogs, and file attachments on your site and update the Quick Search results accordingly.
Submitting a search (hitting Enter) will take you to the Confluence Advanced Search Page.
For more information about how this works, please see Atlassian's documentation on Confluence search.