Hi Shailendra
A BAPI is basic a function module that was designed by SAP to be used by customers to execute an specific business process.
For example, in PP, you have BAPIs to create an order, to release an order, to confirm an order, etc...
Take a look on the following link for more details:
If you call transaction BAPI you will have an overview of the available BAPIs for each area and a documentation with a detailed explanation of each parameter of each BAPI.
BR
Caetano