Crawled properties are metadata information that are extracted from the documents and lists during crawls. Metadata includes information such as author, create date, subject, title, etc.
To enable the Power users to search on metadata, we need to use the managed properties. Managed property is mapped to one or many Crawled Properties. Your search result is based on the Managed properties.
SharePoint 2013 will auto generate a managed property for every new site column, that is either, a full or an incremental crawl, discovered.
To see the Crawled and Managed Properties for site collection in a SharePoint Online environment,
Navigate to Search Settings -> Manage Search Schema.