The Initial page is displayed when the Search Interface is first accessed—before any query is performed (afterwards, the Search page is used). It contains a search box and links above the search box to access the Advanced Search page, Preferences page, Interface Editor (only the administrator can see the 
link) as well as the Help.
See also: How to Perform a Search, How to Apply Saved Queries and Filters, Interface Editor Help > What Is the Interface Editor?