Greetings Pravin,
If your material is not in stock, you will see an unsatisfied requirement for that material in stock. (e.g. t-codes MD04, MD07, CO09). This material then has to be requisitioned (either manually or via MRP) and purchased into stock, then it becomes available and you may issue to order.
An "L" (lager) item is stock material and this will normally be the default item category for Materials that you stock.
An "N" (non-stock item) on the contrary is purchased directly to Order, so it will generate a Purchase Requisition, you then purchase and receive the material directly into Order balance.
Occasionally, you might want to directly purchase a stock material to the Order, you can replace the Item Category when creating the Order.