A search schema defines the configuration of what users can search for and how the results are presented. SharePoint search returns only what is stored in the search index. The results are security-trimmed, meaning they're only what the user has access to see.
The SharePoint search index, such as your favorite internet search tool's index, is populated by crawling sites and the content of your SharePoint Online sites. The content and metadata make up the properties of an item, while the schema's crawled properties tell the search crawler what to retrieve and store.
Crawled properties are mapped to managed properties, and those managed properties are what the search index contains. By default, the SharePoint search service has many managed properties defined. However, if you want to add additional managed properties, you'll need to...
