Tableau Prep is a data preparation tool in Tableau that helps the user to clean, aggregate, merge or other data analysis operations. Tableau Prep is now renamed to Tableau Prep Builder. In short, Tableau Prep is an ETL tool (Extract Transform and Load) that helps to extract the data from different sources, transform the data, and load the output to Tableau Data Extract.
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You can watch this video on Tableau training to know more about Tableau Prep: