Until a few years ago, webmasters could use the “noindex” instruction to request that Google does not index a page. Usually this instruction was given through a metatag called “robots” in the <head> part of a page.
Google would not index a page where it found the tag “noindex”. This meant Google was not adding to its index any link between the words in the page and the page itself (as happens in a book index). Without indexing, the user was not able to find the page with a query to the search engine.