What are the extracts and schedules in Tableau server?

First copies or subdivisions of the actual data from the original data source are called data extract.
The workbooks which use the data extracts instead of using live DB connections are faster and the extracted data is imported into Tableau engine.
Later after the extraction of data the users can publish the workbooks which publish the extracts in Tableau server.
And, the scheduled refreshers are the scheduling tasks which are already set for data extract refresh so that they get refreshed automatically while a workbook is published with data extraction.