Tableau Prep is an ETL (Extract, Transform and Load) a tool that is used to extract, transform, and load data for further purposes. This tool from the tableau suite cleanses the data before processed further using other tools from the tableau suite.
If you are planning to get certified in Tableau, then I would recommend you to take up the Tableau course from Intellipaat which helps you to prepare for Tableau Desktop Qualified Associate certification exam through its 30 hrs. online training course that also includes 7 guided projects which will help you in making you proficient in Tableau.
Also, do check out the Tableau certification introductory video to help you get started.