Google stacking is an method that is used to improve the search engine ranking. Google Stack is a Google Stack is a collection of Google apps or entities such as Google Sites, Docs, Calendar, MyMap, Draw sheets, photos forms, slides, and Forms filled with company data that are stacked over each other (so to say) and released to the available to the public, taking advantage of Google property and power they hold within the Google algorithm and ecosystem.
Google Stacking is an authoritative SEO strategy that permits you to establish links across several Google platforms that connect to different company assets such as your website of your company. This "Google Entities" comprise content from assets such as Google Drive, Sites, Docs, Google Sheets, Google MyMap, Youtube, Photos, Draw and more. Google indexes content found on its own domains and assets quite fast, as well. A result from a Google search could very well contain links the Google assets very quickly.