By default, extracts are fully refreshed. That means that every time you refresh the extract, all of the rows are replaced with the data in the underlying data source. While this kind of refresh ensures you have an exact copy of what is in the underlying data source, it can sometimes take a long time and be expensive on the database depending on how big the extract is.
If the extract is not set up for incremental extract, selecting to refresh the extract will fully refresh the extract. If you’re publishing the data source to Tableau Server, you can specify the type of refresh in the Scheduling & Passwords dialog box. Most data sources support an incremental refresh.
Rather than refreshing the entire extract, you can set it up to only add the rows that are new since the last time you extracted data. For example, you may have a data source that is updated daily with new sales transactions. Rather than rebuild the entire extract each day, you can just add the new transactions that occurred that day. Then once a week you may want to do a full refresh just to be sure you have the most up to date data.
Source: Tableau Community by Mahfooj