Within SharePoint search, there are two key configuration components important to the ability to search for content. The first is called crawled properties. A crawled property is created for every site column that exists within SharePoint. This includes standard, out-of-the-box columns like Created By, Title, Name, etc. It also includes any custom site column created by users. However, a crawled property is not meant to be used as a field for direct searching by users, for filtering results, or for refining a search.
To make columns searchable, sortable, refinable, etc, the columns must be updated within the search schema of SharePoint. To allow users to actively use the columns in search refinement, custom queries, etc the columns need to be connected to the second key component of the search schema: the managed property.