Hello Sambath,

In the simple meaning, Procurement is "The action of obtaining for required products".  On confirming Sale Order or checking minimum stock rules for replenishment, a demand of stock will be generated. So procurement is the process to prepare and process on that demand.  

Procurement is the central place where system decides what to do against demand. Whether to purchase or to produce upon product's routes. The process of procurement is often part of a company's strategy because the ability to purchase certain materials will determine if operations will continue or not. A business will not be able to survive, if it's price of procurement is more than the profit it makes on selling the actual product.

In a nutshell, Procurement = Acquisition process to obtain required materials.

Example : Once you confirm Sale Order some of the Qty may go for purchase to supplier and some of the products may go for to production department. ( In case of if those are not available in Stock ) At procurement, system look for verify the stock & product supply methods in order to make them available and reserve for an Order.

Hope it is cleared now  !