Hello Prachi,
Data cleansing is core concept of ETL (Extract-Transform-Loading). In this process, data is Extracted from different systems then it is passed through the Transformation process and finally Loaded into application/warehouse.
Transformation (or for a moment say it Data Cleansing) is carried out to maintain the data consistency. For example, we have an enterprise running with several departments such as - Sales, Finance, Marketing etc.
Country Name field may have different values in every department (Sales - IN, Finance - IND, Marketing - INDIA). To make it consistent we do convert all these values as INDIA.
Hope this will be helpful.
P.S.: This is just a single example; this is a vast topic.
Regards!
Rahul
