It is called to the pages of your website that got stored on search engines.
This way search engines like Google can prepare a search result before it is asked for. This process is called indexing.
indexed page process
There are different ways to do so but the most common is to create a sitemap of your website which is literally the map of your website and upload it on Google search console which is connected to your account. This way Google gets to know more about the pages available on your website which is very effective for SEO.