In SharePoint Server, the search schema determines how content is ingested from the search index. In Chapter 7, Planning and Configuring Managed Metadata, we discussed how metadata is data about data. That metadata is extracted during the crawling process, including both structured content (derived from properties of documents, pages, or other data) and unstructured content (extracted keywords). The 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 instruct the search crawler on what...